I ordered a 7800 Prosystem from Best Electronics about two months ago. I've been playing it ever since.
I was initially thrown off by the appearance of their website but I can 100% assure you that the place is totally legit. Totally professional in the way Bradley Koda (the owner) handled my email requests, and I was very impressed by the packaging and time of delivery, only eight days from California to Canada.
As far as I can tell, judging from the serial and its mint condition, the unit which I received had been pretty much sitting on a pallet in one of the warehouses acquired from Atari for the past 23 years. I requested the video upgrade for my unit, which is well worth the premium considering how easily it can be plugged into a fairly modern tv.
I tried it out on my Samsung LN55C630 LCDTV and was fairly disappointed with the color and crawling lines, especially in Yar's Revenge, the edges of the pixels crawl like crazy. Initially it was about a 4/10, just bordering on unplayable, but with some adjustments (setting it to Game Mode, adjusting contrast and brightness, etc.) I was able to raise it to about a 6/10. So, if you're planning to plug it into a LCDTV or a plasma screen, beware. Fortunately I hung onto a couple of CRT TVs from the 90's and it's about a 9/10 on the JVC 32" I bought in 1999, almost perfect, really.
I just ordered and received some Atari controllers from them, and I was shocked to find that they're brand-new. The Atari Trakball came in the original box from 1983. They also do an additional service refurbishing their CX40 controllers with improved contacts and the paddles with super pots that last forever; it's only a few more bucks, but I strongly recommend going for the upgrade.
Very impressed at what I received from Best Electronics, I'll definitely be ordering from them again!