Huh, I've literally never heard of someone before thinking it's supposed to be a library that you return books to. Must be different attitudes in different places. I've always seen people treat it as a swap-shop. Take some books you want, and some other day drop off some books you want to give away.
Both are okay. Returning or just keeping, as long as you aren't excessive. You aren't supposed to just take all the "good" ones, no matter if you're keeping or selling.
And there's an unspoken rule that you should probably give back, too. It's a community good, not a charity.
