My Sim is currently living in San Myshuno in the Medina apartments, and I'm having this bug. I've tried resetting the mailbox with testingcheats on since I can't replace it. I've also placed one from build/buy inside the apartment and it behaves the same. I haven't been completely unable to sell things though, sometimes it takes a dozen tries and I've noticed that I have more of a chance of success at the interaction going through if I initiate it while she's walking towards the mailboxes.
I'm having the same problem with my sim from San Myshuno. Whenever I have to ship something from plopsy, I have to click ship a dozen or more times to get it to work. Doesn't matter if it's a knitted item, a cross stitch or painting.
The Sims Team
