I have the same problem with my SLX shifters, and it is a right pain, so i’d get it soughted one way or the other. Have no idea how easy they are to fix either.
My BRAND NEW SLX shifters did this on their 2nd ride… Took them apart to try and fix it, but to no avail.
It seems that Shimano quality has taken a backwards step on their newer shifters, only about 4 or 5 rides later the return spring on the thumb lever decided to stop working. Then it started working again!
As much as I like having gear windows on myself, it seems Shimano believes that almost everyone will take the gear windows off as they don’t seem to work for very long anyway!
Personally, I don’t think the ebayer is trying to have one over on you. Definitely worth sending him a polite email mentioning the problem, and asking for maybe a fiver compensation to see what he says… But knowing what I do about SLX shifters (why do the shifters have to be so poor, the rest of the groupset is quite frankly fantastic!) I’d chalk it up to experience…