Joe, as long as the threads aren't damaged or battered and they have not been dropped causing deformity of the tube, there should nothing wrong with them. If they visually look perfect, they probably are.
A simple check of bore diameter..and internal "roundness" is important..A dial indicator on a lathe can properly check a tube.. While I have bought many used tubes.. a few have been "opened" up.. and not done so well.. I visual glance will usually not tell you anything...
With lead shot.. I doubt if you'll ever wear out a choketube.. Steel shot is another thing.. especially in the tighter chokes..
Some custom tubes were altered to move POI.. Quickly spotted with a dial indicator..it can often be spotted with a keen eye.. One side of the choketube is thicker than the other.. Some tubes were made to correct an improperly threaded barrel.. While the work perfectly in the barrel they were made to correct..they can actually damage another barrel if the rear section encroaches on the barrel bore.. and catches the wad..
All in all.. use caution.. The $10.00 you might save is not worth the cost of a barrel..