If somebody gives you something they don't want, then not too long afterwards you decide you don't want it and stick it on ebay. What's the etiquette here?

I reckon you owe at least some/most of the money to the person who gave you the stuff.

My gf has just sold a bike on ebay for 50 quid that she got off her sister a few months ago for nowt. Her sister was moving house and wanted rid, and now so are we and so does the gf. I've convinced her to give the money to her much younger and much more impoverished sister, and I think that's the right thing to do.