Many games feature side-quests that send the player on grinding item hunts—repetetive tasks with no connection to the story and easily passed over by most. For completists, however, getting them all is part of the fun.

Just as gamers collect rat pelts to level up, they collect the games themselves. This growing obsession has led to a bubble in the auction market, with the most inflated prices commanded by prototypes, unreleased games and rare games still in their 30-year old shrink-wrap. The Nintendo Entertainment System, which once dominated the industry, now rules again—even if its new kingdom is just an obscure corner of eBay.

At the forefront of this movement is game historian Frank Cifaldi, who hates to see old games collecting dust. Cifaldi, specializing in retro-gaming since the 1990s, recently garnered international press coverage for attempting to sell the only known English-language copy of Final Fantasy 2 for the NES.

The tag? $50,000 OBO.

If you think it's crazy, know that Cifaldi picked that price because a prototype cartridge of Legend of Zelda, along with a sealed retail copy, recently went for even more.

"If Zelda, a released game that came out, can sell for $55,000, let's start this at $50,000," said Cifaldi. "I think it's more special. But based on the price that we're seeing, the bar that's been set, let's just set it slightly below that and see what happens."

What happened next suggests that even eBay is clueless about the nature of these rare collectibles: the auction was taken down, Cifaldi says, for violating a term of service "meant to prevent people from selling bootlegs". He is selling the game to one of the interested parties without eBay's involvement.

If the amazing dollar amounts make you wonder at the value of what you have stashed in your storage unit, check out the following shopping list. Assembled with Cifaldi's help, these are—at least for now—some of retrogaming's most fascinating (and expensive) collectable treasures.

Air Raid Atari 2600

"Atari is very bizarre and interesting because this was before Nintendo invented a licensee program, so anyone could make Atari games, and everyone did," said Cifaldi. "Quaker Oats had an Atari games division. We are still finding games that were actually made and sold that we didn't know about."

Back story: There's a lot of debate about whether this game is real or a bootleg title, because rumor has it that its code was built on top of an Activision game. It is the only release by a company called MenAvision, and its cartridge is notable in that it's blue and resembles a bomb detonator. There are only a dozen official copies in circulation and someone attempted to sell it for $5,000 in 2009. There were no takers then, but the price tag soon went into orbit.

Most it's publicly sold for: $31,600, in 2010.

Chase the Chuck Wagon Atari 2600

Back story: It's a promotional dog-food video game made by Ralston Purina. To play it, you had to send in some dog-food UPCs, and be enough of a fan of dog food to want to play a game about dog food. Created to promote Purina's Chuck Wagon line of dog food, it didn't do too great: the promotion was deemed unsuccessful and the unshipped cartridges were destroyed.

Most it's publicly sold for: $760, in 2011.

Custer's Revenge Atari 2600

Back story: In 1982, Mystique tried to establish a market for "adult" video games with this infamous title, in which General Custer sets out to rape a Native American woman.The controversy surrounding this "Swedish Erotica" game—as crude and simplistic as its subject matter—earned it 80,000 sales and a lawsuit from Atari. Though hardly rare, its notoriety helps it command a high price.

Most it's publicly sold for: $149.99 in 2010.

Stadium Events NES

"You really have to get into why people collect things in the first place, but I think people are not as apt to fill in that lost childhood with Sega as much as they are with Nintendo," said Cifaldi. "I also think a lot of it has to do with the fact that Nintendo is still making systems and Sega is not."

Back story: It's the rarest licensed NES game in North America. It was an add-on game for the Family Fitness Pad, which you put on the ground and run on. If you think you are remembering it, you are probably thinking of Nintendo's version, the Power Pad. Bandai's pad preceded Nintendo's pad by about four months, and there were only two games released for it. (This is the second.) When Nintendo replaced Bandai's pad on store shelves, these games vanished, causing them to become among the final titles people get to complete their collections. (You can track down the exact same game with a different title screen for a lot cheaper, and it's called World Class Track Meet.)

Most it's publicly sold for: $45,000 in 2011.

Final Fantasy 2 NES

Back story: Cifaldi is part of a team at Lost Levels, "a group of like-minded people who will front the money to buy a one-of-a-kind game when it materializes." Nine years ago, Lost Levels snatched up Final Fantasy 2 up for a grand. Fast-forward to that aforementioned Zelda auction, and here we are.

Most it's publicly sold for: $1000.

Kizuna Encounter Neo Geo

There aren't a ton of super-rare Neo Geo games thanks to NeoGeoFreak, a company that swooped in and snatched up a lot of the IPs and assets from SNK America when it stopped manufacturing Neo Geo games. That means a lot of unreleased (and unfinished) games have been set loose. But there are still toughies to find out there.

Back story: It was never sold in stores, and only about five of these were made in English (certainly fewer than 15), perhaps for promotional reasons. It's also been speculated that Neo Geo was prepping to do a test run. For whatever reason, it didn't go to market.

Most it's publicly sold for: According to rumors, about $13,500, but it's unclear when the sale went down. In 2010, a new copy sold for $1,247.66.

Rex Ronan: Experimental Surgeon Super NES

There aren't many "hidden" relics for this platform, but there are a handful of lesser-known titles that go for a good price.

Back story: Welcome to the strangest gaming subgenre of the pre-internet age: educational games about the dangers of smoking, how to live with diabetes, and how smoking effects dinosaurs.

Most it's publicly sold for: $349.95 in 2012.

Elemental Gearbolt Playstation

Being barely a decade old, there aren't many high-ticket rarities for Sony's first console. But a couple of oddities merit a mention: first, the Lifespan Learning Series, which go for a song but whose rarity makes for an interesting invesment—given out only to schools, a copy in the open market strongly suggests an act of theft. And then there's this:

Back story: At this point, game-makers knew what they were making were collectable. That's why all those "limited edition" collector's editions of games starting hitting shelves. It doesn't get that much more collectible than this 1998 E3-only promotional giveaway for convention-goers. It included a gold-plated gun, a gold memory card, and the game itself. One fan described the briefcase it comes in as having embossing so detailed "you can even make out the silhouette's nipple." There were only 40 made, for a grand total of 80 nipples.

Most it's publicly sold for: $3,000 in 2009.

If you're not sure whether what you have on your hands is legit, Video Game Authority is an invaluable resource: "There have been expensive high-profile games that have sold for a lot of money that turned out to be bootlegs people made at home," Cifaldi warns.

Satisfied in a title's bona-fides? The place to buy and sell is eBay, of course, through there are plenty of alternatives more in-tune with the market. You just have to be patient—and rich—to get what you want.