I'm sure you've read all the suggestions here to "ride as many bikes as possible". The LBS folks know this. I think if you find a good salesperson, they will understand what you want and get you riding those bikes. I did this recently, and I found it helpful to have them suggest some brands that I didn't know, and different prices ranges, etc. The comparison helps narrow it down. And I can't imagine they have NO used bikes for sale. Anyway, if they're good at what they do, they'll feel confident they can sell you their bike, and will keep at it until you either leave or buy something.

And I wondered the same thing, intially - "won't they be bothered if I want to ride lots of bikes??" Consider car-buying: you spend an afternoon driving vehicles that cost thousands. I've never thought twice about that.