There is 1 set that I do regret buying:
the new tractor, released in 2012. I spent £25 on it and wasn't all that pleased.
Yet, I wouldn't sell it because:
1.I can't find most of the bits because they are in various MOC's,
2.I would never get a lot of money for it anyway, so there would be no point in selling it on unless someone made a good offer