I actually bought my last two lenses (both fairly pricey ones - Pentax 31mm F1.8 and 50-135mm F2.8) from Abe's of Maine. I'm not in love with them but they did ship the lenses safe and sound. They do want you to call to "confirm some details" on the order, and that's apparently when you usually get the hard sell on filters, etc. On my first order, it was indicated that the phone call was because it was my first order, and there was only a brief question about if I needed anything like filters, cards, etc - no hard sell.
After you order, they include a $10 off coupon for later orders (it's just a code, you can use it on multiple orders). Based on this, they had the best price for the 50-135mm so I decided to try them again. Yet again, they asked for a confirmation phone call. I was busy at work and wouldn't have time to call them for a couple days so I sent an email saying that I was under the impression that I wasn't going to half to call every time and that I was not interested in any add-ons like filters.
I didn't get a response, but a day or two later, I got a shipping notice and the lens arrived safe and sound soon afterwards. So I'd say that it's probably safe to order from them, but expect slightly more hassle than from some of the better-respected sites - but you will get what you order.
And frankly, I've heard plenty of horror stories about Dell and photo equipment, and I have a pretty rotten opinion of Dell anyway due to their shoddy laptops, but that's a separate issue.

I have heard more than once about people never getting photo or other non-computer things that they've ordered from Dell.
As for used ones, I've bought a few things from the "marketplace" section at pentaxforums.com. They also have a feedback system there, although it's not used as often as it could be. But you can at least get an idea who's reputable and not by the posting patterns, and you usually get better descriptions and photos of the item than you might on eBay, and cheaper prices. (You're not allowed to "auction" or "best offer" on there, you can to list your asking price for everything you sell.) Some recent DSLR sales on there include a K10D w/lens and extras for $575, a K110D w/28-300mm for $350, etc.