Caveat Emptor absolutely applies, it's a second hand sale so the buyer HAS to be aware that there may be some flaws not immediately visible in the advert. Is it a dick move to not say it's got a gopping paintjob? Probably, yes. Does it interfere with the function of the gun? No. As such it's fit for purpose and the paintjob only affects the potential sale value (which is probably why it didn't get mentioned but it may just be that the seller didn't think it was so bad as to be an issue).
I do agree though that repainted two tones should be declared as such, but in this case as I said earlier, you can clearly see it's been repainted at some point from the advert pictures.