Hey Hive account@oldtimer SBD transactions are almost instant through Steemit so once someone sends you the SBD for a product Steemit will guarantee you receive it. Shipping costs are usually covered by the buyer but if the seller wishes to cover it they can.

As for guaranteeing shipping our current solution is steem reviews which allows buyers to review the seller. If the buyer does not recieve their product they can leave a negative review warning future buyers. We do plan on having better resolutions to this in the future.

-Iris