I've used a combination of buying new & used, although most of my purchases have been used. I always make a phone call when I'm buying anything over a couple hundred & when someone wants to buy my gear, I include my phone # to encourage them to call me. The phone call gives you some insight into the person you're dealing with & although it's not about trying to psychoanalyze them, you'll get a gut feeling whether or not you should make the deal.
As others have poignantly brought up, packing is of major concern. No need to go over all the details mentioned but do make sure the seller will at least double box, unless the manufacturer is known for high quality original packing material. Offer to pay for the extra materials & the 3-day service does make a difference too.
As for payment, although I've never used escrow, that is one option. I also don't use paypal & although they have not personally burned me, I do know someone who has.
Timely responses to your inquires needs to be touched upon briefly. Lets say you found amp XYZ123 you really want & are the first to email the seller & say you want the amp, are committing to it & want to consummate the deal ASAP. You hear back from the seller 3 days later & they are wishy-washy about details, when they could ship, blah blah blah. Please quickly move on to the next seller, as this is indicative how the deal will go down, if it ever does. Most of the time when a "hot" item is listed, the emails start almost immediately & the seller is ususally ready to respond. That's not too say some excellent buys could be advertised for days w/o an inquiry, as it all depends on what you're looking for.
One more thing to add to buying used. Try & get the SN of the gear being sold & the better sellers will have them clearly visible in a photo, have it printed in the ad or will say email them for it. Stay away from those that refuse to give it. You can then check with the manufacturer to see if the SN corresponds with the description of the piece & also to get a feel about the kind of customer service you could expect if you needed service. There are some great threads in the archives about excellent customer service & also about not so great service. You can also check out the archives at Audio Asylum, as many 'philes frequent both sites.
Sorry this is getting long & although I only intended on writing a few lines... well, here's one last topic. Buying new. I bought my spkrs. new about 2 years ago from my local dealer. Unfortunately, they've developed a cosmetic problem & need to go back to the factory. My dealer came over to my house to look at them & this week the rep will come by & pick up the spkrs. to ship back. At the same time, the rep will leave me with another pair (brand unknown) so I can have some tunes. If you have a local dealer who will work with you, answer your questions & let you take gear home for demo's, they can be invaluable down the road. This is not to discourage you from buying used but only to suggest using the dealer & paying close to retail isn't always the least attractive way to go. Sometimes you have to buy from a dealer, as the factory doesn't sell direct & the item is either so good or new on the market that used ones never come up for sale. Also, there are some manufacturers that ONLY sell direct to the consumer, so all your concerns would be on a direct basis. There are some excellent dealers around but you have to figure out which ones you feel comfortable with. Again, there are past threads relating to this & many other topics.
Good luck with your search. Do let us know how your journey goes & what you wind up with, even if it's a year from now (or two).