Answer: potential buyers must be able to see the watch and understand its qualities
Step-by-step explanation:
The condition that is necessary for an exchange to occur between Barry and the buyer is that the prospective buyer should be able to see the watch, and also understand the qualities the watch possess.
This is important as the buyer would be able to determine if the watch meets the qualities he or she is looking for and also if the watch is worth the amount the seller wants to sell it. Any further questions regarding the watch can also be asked from the seller.