It's pretty simple - once you look up a game that's in your collection, you can click on "add with details" and there's a checkmark there to mark it for "on sale." Basically the same as trade... I think the trade/sell value is based on whatever communication you have with the person on the other end of the deal.
Also, the other way to go about it is adding all your items to your collection, then when you're on your collection page, just hover over the item and select the pencil icon. From there you should be able to edit info about the item and mark it for sale there.
Also:
Once you got your sell list going, you can hover over your item(s) to see if there's a buyer match (someone who's already listed that item in their wishlist). From there you can
message folks look at folks' collections. Same goes if you have a wishlist and click on the red number (assuming that it's not "0"
) under Seller Match.
