I'm looking for opinions of what is proper and fair for parts that are sold and bought out of the OSO swap shop.

If you find a part you are looking for in the swap shop agree to a price (plus or minus shipping) and send money to the person for said item i have always been of the opinion that when the money has been collected by the seller that they are responsible for getting the parts sent out in a timely manner. Along with this i believe that tracking numbers should be forwarded in order to make sure of delivery/ issues.
When does the liability of making sure that the parts are delivered to the buyer end for seller? And if there is an issue with the parts not being delivered what would be considered proper protocol?
Just trying to get a feel for how the majority of people would think of this situation...

Dave