Ages ago I went into MAC in House of Fraiser in the Metro Centre and said I was really interested in prep and prime, she just went over and got it out of the draw, but I hasn't finished talking and didn't say I wanted to buy it. So I explained that I get breakouts and wondered if I could have a sample of it to see if it works well with my skin before I buy the whole thing. 'No we don't do samples' and looked extremely pissed off. I just walked out. It's so hit and miss at that counter.
The MAs from the Pro Store in London are usually nice, they are quite wary about giving piggies samples but they never refused other products samples (strobe cream, microfine rifinisher, foundation, etc)