where to trade non steam left over bundle games?

https://www.steamgifts.com/trades

https://www.barter.vg is the main one that comes to mind. There are plenty of trading sites, but the majority are for steam only.
There is of course, as noted above, also the steamgifts trading section, but it'd be worth noting in the title that they're non-steam.

You have a 2 month old trading post which is "[H] Steam, Origin & Uplay Codes [W] Wishlist & Offers". You know this already.

barter.vg mostly focuses on steam trades, but it does have support for several other platforms, and a lot of control in managing your trades.
Both barter and steamgifts allow you to trade non-steam for steam, so those are good options; just keep in mind that GOG games tend to trade a bit worse than steam, origin a fair bit worse, itch.io at minimal value, and uplay is practically worthless. :X

You can trade GOG games in various places at good value (especially GOG-for-GOG, as per their mystery key events), and there are niches for origin and uplay (platform-exclusive games do often trade well, as well), but itch.io tends to be very low value no matter where you trade. If a game is itch.io-exclusive, it may trade a bit better, but ones with steam versions are rarely considered worth trading for.

I just recently started trading and set up a profile on Barter. Despite looking a bit chaotic, it's actually pretty simple to use. I also successfully traded a UPlay copy of Rayman Origins on there.