It's completely up to you RES, there are no rules about that. Some guys give a three-day inspection period (usually on complete firearms), but it's not required.
If you feel pretty confident that he'll be happy with the barrel, you can agree to an inspection period. If he decides he doesn't like it, somebody is going to take a hit on the shipping...probably you (X2). So make it clear up front whether or not there is an inspection period... and who gets nailed with the shipping in case he doesn't like the pipe.