I know there are posts when you search on this topic, but most are dated
and I was looking for some recent information.... on places in center city that will buy used books for a couple dollars.

Does not have to be alot, I know people mentioned Book Trader on 2nd
Is that still a good option.

Otherwise I was going to donate the rest to the library,
no ebay/half is not an option.

Book Trader only buys them for store credit.
Long in the Tooth on Sansom is a CD store, but they have a small book section and I know they buy for cash. (Call ahead to see what they're looking for.)
There's another place on 22nd right below Locust -- I think they buy for cash-- looks like more scholarly, serious books.

I know there are a half dozen others. I'll have to post again when I'm more awake.

Not in CC, I know, but the Last Word, on the 3900 block of Walnut (moving soon to 40th between Walnut and Locust), will pay cash for books but are very picky about what they accept. It's a bit of a humiliating experience to have your books turned away.