The owner of a Board can delete the Board, but not leave it. Anybody who joined the Board can leave it. The steps are almost the same for both. You'll only see the options that will work for you with that Board.
- Log in to OfferUp
- Go to Offers, then Boards
- Select the Board you want to leave or delete, then tap Edit
- To delete the Board, tap Delete, then tap Yes
- To leave the Board, tap Leave, then tap Yes