You just issue IOUs. It's just an asset backed by a promise, just like mastercoin or counterparty. You can have it mean whatever you want.
Suppose I issued an IOU and someone bought it, but I don't fulfill my promise (scam in other words). What happen next (to me and to buyer)? My IOU value drop to zero?
But how a buyer can "buy" my IOU? with XTS? or with BitsharesME shares?
It's up to you, the customer, and the public to resolve un-honored IOUs. You could also theoretically do something really fancy like have a piece of hardware that requires ownership of the asset on Me to work, so then your asset could effectively be traded as a title for that piece of hardware.
Buyers buy the asset on the exchange within Me, using other Me assets or the core Me asset (XME?)