It could be as innocent as you messing up the encryption, or them messing up the decryption. If it happens every time, they could have lost their private key and are too embarrassed to admit it. Doesn't matter though, I wouldn't send them my address in plaintext. They need to own up to losing their key or I would shop elsewhere. Of course, it could be a compromised account. There's no way to tell for sure, unless something obvious happens like buyers start getting arrested. Also, people don't realize that PGP offers more than communication privacy. In a pseudonymous community like this, it is your identity. It is the only reliable way to prove you are you if something happens like your account is compromised, or you communication with people through out of band channels. Backup up your key in 3 places, because losing it is a huge pain in the ass.