Don't lie - "I don't know, but I will look into it and get back to you" is a perfectly acceptable, professional answer.
If I ask for a quote on something that is not in stock and you take more than a few days getting back to me - I'll find it somewhere else. Remember, it is often faster for me to order accessories on-line from across the country and get them delivered to my door than it is for your sales rep to get back to you with a price, let alone how long I am going to wait until you consolidate enough orders from that manufacturer so as to make your shipping cost acceptable to you.
Know what you are selling. If I buy a BP/w, don't forget to recommend a crotch strap and tell me why. If I am getting a tech setup, don't try to sell me retractors (no matter how good the markup) - offer me a bolt snap solution. It really helps if you have dived the gear that you are selling so that you can explain to me why I do or do not need something.
Don't try to push me on the last of your old stock so as to clear it out if that is clearly not what my diving needs are.
Educate me on my diving needs, but don't talk down to me.
An up-to-date listing of what you have and your pricing might encourage me to shop at your place on-line. Do you think I really care if it is shipped from NY, TX, or TN? DON'T have a web link to your "on-line store," the only contents of which are links to Aqualung, US Diver, Zeagle, etc. - I can find these manufacturers just fine using google.
If maintaining a website is too tough, send out a pdf flyer with real prices on it once a while.
If you sigh, roll your eyes and shrug your shoulders if I ask you to price-match someone else (or at least come close) - you have lost me as a customer, forever. Giving you the chance to come close to an on-line price is not something that I do for my benefit - the on-line price will be better. I am trying to get you to come close so that I can support you while not getting totally ripped off. I am throwing you a bone. Don't shrug and walk away.