To be honest, I have sold old magic DVDs on ebay in the past including ones that have gimmicks.
Here are some reasons as to why I did it:
I bought a couple tricks where they turned out to be absolute garbage and effects I would never perform. So I try to get my money back by selling it on ebay, and in that case, I feel no guilt for the artist since he's the one who put out a cruddy DVD/trick.
If I have some old DVDs that I've learned everything on it, I'm gonna sell it since I no longer need it. Let some other kid who's just starting out buy it second hand for a cheaper price without breaking the bank. Plus I get a little cash back to recycle into buying more effects.
Is it wrong? Probably, but I feel that most effects that come out are overpriced and this is a way to get by.
I've sold gimmicks on eBay before.