Go into game and open your in-game mail. Click 'compose', then click 'attach'.
(Then send me money. No, wait.)
We can already put items up for delivery on a board that one other person can see; that's what the mail system *is*.
Create a series of "inboxes" that everyone can see, one for each general type of item. And add "Pay X to receive this", where X is whatever the seller wants to ask. Only allow one stack of identical items to be attached to each 'letter'. Have the 'mail' self-delete if the attachment has been received.
Probably reduce the 'send mail with attachments' fee to 25cr, and have the mail and the fee returned to the sender if it's not received (bought) after a certain time.
It would be a little clumsy, but I suspect it would also be dirt-easy to implement, and miles better than the present setup.
On further thought, you'd need an "inbox" not just for every type of item, but each star value within a type. Otherwise, you'd go mad, rather than just going shopping.