Re: "what not to do"
I volunteer in a charity shop on occasion and there are some who will try and haggle with you. Pretty well everything in there is a bargain, even the new stuff we flog.
I use the ‘I’m sorry, I’m not authorised to alter prices’ wheeze. I do quite often mark stuff down but only when instructed to do so on the range and using the criteria (usually date put on sale) specified.
Also i am allowed to determine the prices of some things and the range of things is being increased all the time. But don’t tell the customers that or they will all be at me.