I personally think an option to filter collection by category would be nice

List the current lowest value for a listing of the game to the right of the entry, for example:

Nintendo Switch - Super Smash Bros. - $30.00 

and then be able to click on the price value to see all the game listings in the marketplace

Some Games have a separate listing for Player's Choice/Greatest Hits.
For instance, Banjo-Kazooie's N64 player's choice trends 20% cheaper than the original N64 release.
Metroid Prime has only a single entry, and seeing as it is trending upwards, almost doubling in value in the last 5 years, it might be important to see Player's Choice variants. However, is this a limitation of what you can see on the ebay listing?

The reason I'm posting this at all, of course, is a copy of Auto Racing for the Mattel Intellivision.
The copy I found in a 50-piece lot was a Sears Video Arcade / Sears Tele-Games cover variant for sale at Sears. It works on InTV and is the same game program by Mattel.
It appears as though the Sears variants of InTV games trend higher than the default InTV versions. For instance, the Sears maroon-box Tele-Games version of Auto Racing has completed listings at $13-$14 complete, while the Mattel version is closer to $5.
This is true for most variants. Football, for instance, has completed listing for $30 for the Sears version but Trends around $9 for the Mattel version.

There should there be a more robust variant selection tool.

Rather than delete Spyro the Dragon from my collection and re-add Spyro the Dragon [Greatest Hits] as a different game entry entirely, perhaps "Greatest Hits" should just be a checkbox in a "Variants" section when adding.

Another quick example, I have two copies for Need for Speed: The Run on PlayStation 3. Both are English US releases, but one has a different DLC car included (later release on the shelf) with said DLC car replacing the original cover art's car (but otherwise look identical).

Other examples:
Ocarina of time Gold carts, vs. Grey. 
Majora's Mask pre-order "holographic" lenticular label vs. the default release label.
Version numbers, for instance, Super Smash Brothers Melee versions 1.00, 1.01, 1.02, etc. for compatibility with Action Replay or glitch use. Different versions of other games are relevant for speed runs.
Original copies of Lumines could be used to jailbreak PSPs who had PSP firmware 3.5+, and commanded a high value, while later releases of Lumines patched this (no longer relevant today, but still an example of two versions of the same game having a different price).

Obviously tracking a lot of these prices are not realistic considering ebay listings rarely care about every little detail, but it would be useful for variants with large differences like the Sears/Mattel InTV carts, and a cleaner way to list Player's Choice/Greatest Hits copies.

Especially as Sony gets ready to launch their new "Only on PlayStation" cover variant line.

We need to be able to add other currencies to the "my cost" / "price paid" section... I live in Canada but when I want to add the price I paid for my games the only option is to put it in USD... so annoying. The conversion already exist elsewhere on the site, why not for the "my cost" section?

Allow users to enter a cost in their preferred currency instead of only in USD.

I love the ability to add games / consoles / accessories to my wishlist.

I just wish there was a way to filter it down when I'm looking at it to just a particular console system and/or a particular genre type. For example, if I wanted to look at my wishlist and see just the PS1 RPGs on it, or just the Sega Genesis sports games, etc.

Eventually it might be good to filter by a price range and minimum # of historical transactions that have occurred with it too (although that's less important).

It looks like this is already being done in the collection tracker, so would be nice if it was also done to the wishlist as well.

PS3 and PS4 are region free.  

Add coverage for Japanese PS2 games. 

It's cool that these games are included, but they often clog search results and they're almost never what I'm looking for. For example, if you search "Mario GBA" you get 29 results, 10 of which are Not For Resale titles. That's over 1/3!  Maybe give the option to filter them like is currently done with region. A little option that says "Include Not For Resale Titles" and let people pick. 

i have a bunch of these and this would be really cool because its a part of video game history

Just started a collection of Nintendo power and would like to be able to mark with or without poster.