I've bought demo speakers a number of times and never had any problems. I say go for it, if it's a good deal. If they don't sound like there is anything wrong with them, they will most likely be fine.
One pair of speakers I bought was sold at a rediculous discount because the grills were damaged - the little plastic retainers were broken off. It cost me $1.50 to repair them.

I'd say I got the better end of the deal.