This is not what real karma is about.
You give not expecting to receive, nor to control what the recipient does with what you give.
You have no reason to be offended if they sell it, if that is of benefit to them then your original intention was satisfied.
If you wished to retain control then retain ownership.
Simple.
If I was given something and then used it to finish off a bike I then sold, would I have to estimate the value added by that item and return the same in karma to appease the angry mob?
Once you have given, you have given.
I do understand how it would seem were someone to do this routinely with such things they receive. That would be twattish, and you can rest assured this will attract no good karma to them.
When I give something to someone it is for the pleasure of giving.