Once it leaves your hand, its theirs to do with as they please. That said, selling a gift plug, unless really in need, is sort of low. The only time I would get a little pissed is if I gave a plug to some who for testing purposes, and they sold it without giving feedback or noting when they sold that it is a proto and may not be sealed. Would hate to think that I gave someone a plug for testing that, while the action is good, it was not sealed, I used cheaper primer/paint/clear to get it to them quickly, and figured I would send them a good one once I got feedback. If that plug goes out to someone else who thinks it your normal stuff, you the builder stand to lose in the long run. That is, you got no feedback and someone may have a substandard plug with your name associated with it. And no, that person would not get another plug from me
Personally, I prefer to fish the ones I get (even tadpoles). In rare instances, I have given traded plugs to someone else. Only happens in cases where I know they will use them and I have not used them in a while.
Jigman