As far as the checks go i beleive if there are two checkmarks it means they have clicked on it, usually indicating having read your text. Its nice to have this information especially since we dont always get an acknowledgement.
@Sherry13--That's correct. Within the OfferUp app, if a message has 2 check marks that means the other person has read the message. Helpful when seeing if your messages are being received/read. Thank you!!