Make sure to add a '+' tag on games you already own (and wishlist them, of course.)

We have to search for the game and do it individually for each game? That's quite a hassle. Can we have an option to include games we already own in the trade window?

Not before I start working on the profile options thing...
And even then I don't know if it's a good idea to add this and that option to the page. What I'm thinking of is maybe a checkbox saying something like 'force showing all games' or something on offer creation pages.

I had a similar issue when I wanted to changed store tags for many games at once. What about bulk select (in a similar fashion to bulk add). The user pastes a list of games and they get selected. It's quicker and less frustrating than doing one-by-one  :P

Hmm, the problem with 'select all' on lists is that you're just one click away from 'delete all'. This has the potential to clutter the database with empty IDs and I'm not sure it's currently set up for that. I'm trying to keep the table lean & clean, so I don't end up with a site that takes several minutes to match games (I'm not pointing fingers :P), so this is something I've been avoiding for now.
Of course I could also simply disable the Remove button when more than 50 items are selected, for instance, but if you selected them all manually, it would be quite annoying to see the button is disabled, and not know why.

Anyway... Just not my priority I'm afraid.

I think you should add it because the way it is now when we make or counter an offer we have to open a separate tab to look at all their tradables and then open more tabs to whitelist the games which is pretty inconvenient.  Showing all their games in the trade window would be easier.

I just don't know what kind of UI to present for it. Any mockups..?

You could do something like Barter with collapsible menus.  Or you could list all the games and then have tags/colors next to games you already own, games you have available for trade, etc.

The things I took from Barter back when LT was a replacement project (i.e. when Barter was no longer accepting logins and had lost most of its users) were the things that made sense and were usable. Everything that's not like Barter is just me saying at one point, "okay, this can be improved."[1]
So, yeah, the fact that LT doesn't have collapsible menus for offer creation is a deliberate choice. (Just like the fact that there's no 'Fail' button on offers for now. Although I'll have to handle that as well ASAP.) The main reason is that most browsers don't bother to auto-open them when you're searching for a game name in particular, so at worst the browser will act as if the game wasn't found, and at best it'll show you the game, but in a collapsed list and as such it highlights an invisible part of the page (e.g. long reviews on Steam.)

Currently I'm leaning towards a dropdown indicating what games can be listed. Dunno if it'll be remembered across users. Also dunno if I'll reload the lists through Ajax or directly the page. (Ajax is longer to implement. Although I can reuse most of the code I wrote for my Ajax implementation of the bottom half of the offer creation page.)Yes, I know, I'm already doing it in some situations, and if I end up showing all games I'll always do it, but it doesn't change the fact that page loading can get really, really long when one of the parties has a huge library. LT is hosted on a top-notch server, it's fast as hell, but it still has to send you the data. ;)

1.	As evidenced by the fact that I haven't taken a single thing from Barter since I opened, and Barter took a few ideas from me. Didn't bother to credit me but I'm mirroring his game database so let's just say we're even. :P

I guess you could show only wishlisted games by default and then have a dropdown menu or checkboxes to show/not show  games you don't own, games you do own, games in your tradables, and wishlisted games (since some people have huge wishlists also). I'm sure you'll figure something out. I have faith in you :)