When browsing games to buy, have an icon to show what you have in your collection already so you aren't buying duplicates.

Where would this icon be shown?  We show that already on the search results, the browse by console results, and the individual game pages.  It is an orange button on the item instead of a blue one.  

Using the lot value calculator or any variant. 
https://www.pricecharting.com/lot/5wkxpvznkp

There should be a toggle to switch between if the game is loose, CIB or new, and when toggled updated with price relavant to current state of selection.

Correlated with what we want to know. Do we have this game from before? And preferably, what state.
So I pick Banjo-Kazooie and get the price CIB as that is what is for sale.
I see I have BK from before, and if it is CIB or box+game. 

Last bit is more tricky, as it relates to personal evaluations.
But over time it should even out.

I have started to enter what state my games are in.
B - for bad (usually includes notes)
A - for average 
G - for good (no deep marks, 
VG - for very good (which would mean (almost) flawless to my knowledge.

I also use - and + as modifiers to indicate whether or not it's up there. 

Example: If I see a G item, particulary G+, I don't bother to replace the one I have or match them to see which is best.
Then only a VG item will be used to replace the current items place in my collection.
Saving a lot of wear and tear on taking things in and out of boxes and covers.

In addition, if used extensively, the system could add on an opportunity to upload pictures of my own items.
Haven't worked out a common system yet, but I would say Im using an 850lm Ikea E27 bulb at a distance of 4-6 inches to evaluate my items. Moving the item allows me to see how the item has flaws and how deep. 

This system will not replace grading companies, but to a certain degree it will reduce the need for them on lowerend items.
The last bit is that the average evaluation of ALL my items would give an average of my ratings. Lets say my average rating would then be 74%. (I can provide my math on how to get to 74%). The sum of ALL evaluations might be at 68% or 82%. That would give a me prompt to reevaluate my evaluations of my own items. A button provided which states, Certify quality evaluation (and if checked, gives higher relevance on my collections in the average based on the size of that collection).

So I check - Certify, My collection now has a 3x the relevance in the averages.
I have to recertify every month or two. (Easy update, just with another overview on my collection vs the average).

Quantified this would give AWESOME indications on how well the state of your collection is.

Later we might have second-verification if an item is sold between two users on this site.
(Like chess-elo ratings) giving certain users a higher relevance based on how well they hit.
 
Heck, it might even be a thirdparty opportunity here to mediate this for a small fee. Not as a decision, but to make parties reach a common ground.

Very interesting ideas.  I can definitely see the value in the first part when buying lots and being able to change the condition for each item to get a more accurate value for it and being able to avoid buying duplicates.

Your system sounds really good for your use case.  Each user is slightly different though so I'm not sure if that kind of system would apply to every user.  Very creative solution for you though and I'll think about if there are ways we could apply things like it in the future.

Thank you sales. I'd also have some other ideas which may pertain to a commercial cooperation for the site. Where can one get in touch?

The idea above gives a health rating for your collections. Since I collect across platforms I see the system being usable for almost any type. Even cassettebased ones. However, I did forget to bring a particular status, which is that of the box. 
Ie. G+ cartridge in a A box. In my offline tracker I even included if carts have batteries, or special statuses for things that contain consumables (mostly meaning batteries).

Id rather do most things in PC than in parts online, parts excel.
Today I print out the excel with the grading field and battery field indicated to achive ultraspeed browsing at flea markets etc.

You can contact me via email at sales@vgpc.com to discuss any other commercial or business development ideas.

i know this is an old post but I was thinking of this same idea. When you add say just a manual to your collection with the intention

of getting the game later on, it can be difficult to discern what you

have unless you go to your collection. I tend to just search out the

game on the home screen (as it's just a little bit faster) which usually just states "In collection" if you have it.

What I've been doing is tapping the wishlist button to give me a hint

later on that I still need the game if I just have say a manual. But I

was thinking, it would be nice if there were icons next to where it says

"In Collection". Like have a book next to it if you have the manual. Or

an icon of a disc/cartridge if you have that, or an icon of a box if you have the box and any combinationof the 3.

Thanks for this feedback.  We might have room for that on larger screens, but I'll need to think about it more and the possibility of it from a tech perspective too.  

If you click the "In Collection" icon, that will take you directly to that item in your collection so you can see was is included.  It is an extra click, but it will give you the answer right now.