Currently the graph displaying the value of your collection over time includes new additions to your collection as increases in value to your collection
My suggestion is to have an optional graph which shows the growth in value of your current collection over time, letting you see how much your current collection was worth x years ago as if you had always owned the collection.
Allow me to import a CSV from TCGPlayer with the Magic and Pokemon cards I already own and track there.
right now it shows all kinds of crap mini systems and model 2 and 3 mixed in with model 1. Same with snes and nes. Also its showing less expensive variants like the sega gt/jsrf combo mixed with the good jsrf. Then also regular xbox version or peter jacksons king kong driving down the price of the 360 version.
Paypal is an incredible hassle to use, to the point where even ebay has dropped them from their services. So it'd be appreciated if there was alternative options, one that comes to mind immediately would be Venmo.
A button available under your collection that allows you to export an item in your collection (with all photos, relevant data, etc) to a new identical entry on your selling page.
Let us know what features you want on PriceCharting.