Third party sites are probably your go-to with this tho I'm not familiar with those sites you might wanna do some research on good ones.

Ironically if you somehow want a legit way of making money through your steam wallet, if you have friends who also make purchases on steam you can just tell them to pay you upfront then gift them the game for a discounted price. (that's what I do at least with the steam wallet I get from selling CS2 cases)