... Buy from your local dive store. In my experience, most local dive shops are able to get pretty close to on-line prices.
Maybe, but usually only if you ask them to price match. In my experience, most shops will be more than happy to charge you full retail even if they know you could get a much better deal.
.... But even if you pay a little more, you're getting service you can't get on-line.
Yes (..and No).. Many shops send your regs off for service anyway.. So for that.. I might as well send them off myself and deal directly with the service supplier. There are several very good options for mail-in reg service, and although that mandates more planning, it puts you in control of the process and opens up brand choice by a lot.
Of course.. You cannot buy gas online.. but if you are a vacation diver, you are not buying gas locally anyway. And you might have other options for gas fills anyway if you look. (ie clubs, fire stations etc).
I am not against local shops, and I typically weigh the cost benefit with most of my significant purchases. I do try to support the local business, but there is a limit to how much one can take. ie: If you can afford to spend $500 on a particular item you need, are you going to buy an inferior product locally just to help out your LDS? Or buy the higher end product from an online supplier.
Re: used gear.. This is a very touchy subject, and depends a lot on the kind of customer/diver you are. I would never buy something used unless I had a 100% understanding of how it works and how to test/trouble shoot it. I have purchased used gear that was a tremendous bargain, and I have been ripped off by people selling junk. It's all about being educated and knowing what your looking for.