I think a key concept is what constitutes "Clearly unopened" and "clearly unused".
Having seen packaging, other than some melted seam blister packs, etc, its not really THAT obvious as to if the box had been opened or not. Perhaps if I inspected the box, and found a tape seam that could be proven to be a factory installed tape closure, with a security layer that delaminated if un-adhered, etc...vs the same thing but not delaminated, etc.
In short, I think its a bit short sighted to assume a clerk can immediately tell if a package was opened and/or the product used or not...in most cases.
IE: They don't have the back story we have when we walk into the shop with the item. They see a return, and start asking the questions they have to ask to process a return, as in do you have the receipt, is there anything wrong with it, etc.
The stores sometimes track the responses, such as if it was too large/small, wrong color, defective, a gift, etc.
Essentially, if you are returning something, there's a reason, and they typically record it on the return forms, etc. So, you bought the item, never tried it out, and are returning it...why would you do that?
The answer is what they are trying to get, for the form.
So, while
we might feel smug and smart knowing in advance that the box was unopened, and the sex toy for the dungeon was never used,
they really just end up feeling even worse about being a clerk than they did moments before you came in and tried to humiliate them/impress them with your superior wit.
A witty good spirited repartee between the clerk and customer is fine of course, but if that's really just the customer making the clerk feel stupid, on purpose, its mean spirited in my book.
Rattling your friend's or cohort's cages is different, as there is a level playing field. The clerk cannot make a snide/revengeful come back w/o jeopardizing their position. The clerk is trapped, and has to take what you dish out.
So, you guys can think of all the ways to make a clerk feel stupid, and go forth, and feel as smart as possible for mentally outgunning the guy behind the counter...or, just suck it up and acknowledge that the poor guy is mostly just going through a standard return process.
It just smells of bullying to me.
If the reply is simply funny, but not insulting, sure...fun is fun.