Worst thing to say to a salesperson.

One thing that will always annoy any salesperson is if you know even the tiniest bit about the thing you want to buy, be that bikes, cars, computers, technical stuff in particular.
If they knew anything about anything technical they'd be doing something useful like service,repair, etc... but because they don't know their RAM from their elbow, they're stuck here selling, secretly envying the technical people who DO know what that button does.
Anytime I had to buy something I have to read up on it myself and decide for myself what the best thing is for me. Then walk into as many shops as possible to check out the price and usually get it for half the price online. It's do or die sucker, no one will do you any favours. Compete or go bust. It's that simple. End of.
So for me it would be:
Excuse me, can you tell me if this Digital8 camcorder is capable of playing back 8mm and Hi8 tapes and does it have video digitising on the fly through firewire?
Answer usually "mumble, mumble, s'arealnicecamcorder, mumble, mumble" or "I wouldn't know about that now" or "ah jaysus nobody would be wanting that round here now" or of course from the better type of salesperson "But OF COURSE it does that, buy it now, take it home and if you don't like it bring it back" (I respect that, a good dose of confidence and cheekiness, always works).
Once the salesman is a spotty teenager, especially girls, avoid, turn around, leave while you can.
They're only there to make a few bucks and have ZERO interest in you, what you want, the product they're supposed to be selling, the shop they work for, their boss or anything beyond their phone and iPod.