This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
Be honest with the sales person. Set some reasonable request
and ask them to meet it. For example, say that you will buy
this if you can try it at home first and listen to it
side-by-side with a piece from another store. Alternately, say
that you saw the same thing at store Z for $xx less, but you
will buy it from the guy if he will match the price.