Intervening between a customer and sales staff is considered bad form, unless the advice being given is clearly life-threatening. This is a case where you need to bite your lip and move on.
A "buddy" of mine, really more of an acquaintance really, was in the general store in our town that has always had a small gun section. Not the kind of place I do business with anymore because of shady, underhanded things that have taken place but regardless, in the following scenario, the owner and his partner were in the right and my buddy was in the wrong. Me and my "buddy" are in the store and some guy goes over to the gun counter with a old Luger and proceeds to ask the owner what he would give him for it and before any real discussion can take place between them buddy jumps in and says "I want it, if these guys don't take it off ya I'll be outside and we can figure out a deal".
Well the owners partner doesn't find this very impressive and says "you need some cold fizzy water on your head boy?". Confused my buddy is like "huh?". The guy behind the counter tells him he is way out of line coming over to interupt their business, asks him "where are you standing right now, and just what do you think you're trying to pull". Buddy all flustered and confused starts ranting to me as we leave about "F them, I bought 7 guns from them this month alone, they will never get my business, yada yada".
He didn't appreciate what I had to say, I told him that just because they are shady as all hell and people I wouldn't deal with, he was out of line jumping in the middle of that deal. He couldn't see the sin of it. I told him, they probably wanted to offer him next to nothing on the gun, give him the run around that it would be a tough sale and they'd be lucky to sell for $150 and make anything on it, you jumping in there gave him the option to hear their ridiculously low and insulting offer and reject it based on the very eager, happy to deal customer waiting outside. I said no matter how sucky these guys are, we were in their place of business and you interfered. I said next time you might want to just skulk in the parking lot and try to make your approach on the seller less "public" if you're absolutely insistent on poaching their business..... he just didn't get it. Very dim. Oh well.
I can't count how many times I've seen somebody come in with something to sell and bit my thumb going "damnit, damnit, damnit".