hey Geyser - have to say that I unequivocally and very emphatically agree full-on (is that definite enough?!) with CorsairMac and all others who recommend a professional bike fit. Can't emphasize enough how important it is to have that process not be avoided, shortened or not done properly and, from your account of the guy at the bike shop, you definitely did NOT experience any part of a proper bike fitting experience! If you buy a bike that doesn't fit you, yes then there are some ways that you can tinker with it to improve it, but why not start out with a better frame fit, so that you're comfortable on your bike from the start? That way, you'll continue to ride and enjoy it, to boot - all good goals, eh?
You're totally right to doubt that salesperson, as I don't read any expertise in what you told us about his "customer service" to you - although I would hardly call it service at all...keep asking questions, keep learning about what works best for YOU and keep insisting on getting better served by so-called cycling professionals, as the ones who truly have expertise wouldn't make that kind of a recommendation to you after only eye-balling you on/near a bike for a few minutes; that's just bogus, IMHO!
Tailwinds!
Mary