Adcom - A Good Value?? Auctions risky??

Look, in a way you can't have it all. 200 watts, good to 4 ohms, under $800, warranty.....(however you did not mention your other equipment, sound preferences, etc).
if you're buying used, there's nothing wrong with Adcom in general. obviously a bad unit is a bad unit, no matter what brand it is, so only deal with reputable sellers, otherwise it is a gamble as you say.
it is hard to get 200 solid and clean sounding watts that are stable into 4 ohms and lower without spending about $2000 retail (the Marsh amp A400 I believe is a good design and made by cheap labor/assembly costs in Thailand so the price is relatively low). The 5802 is a little less than that retail.
Sean's advice though is good....and ask a lot of specific questions about your intended purchase, like has it ever been fixed, modified, blown up, whether the seller is the first owner, why selling is also sometimes indicative of condition though not always easy to get an honest answer.
also big items are expensive to ship and they need very good packing.
parasound is also a good amp, with high power for few watts, nad, etc.
good luck

My own experience with Adcom is that they sound pretty much like sound reinforcement amps. Coupled to goodish speakers they're likely to be bright and tight--design considerations to help them cope with slow, tubby road speakers.

They wouldn't be my choice for music.

That being said, some very critical listeners use PA amps to drive passive subwoofers.

But if I were going to do that, I'd go to Mars Music and buy a Crown K2. Dead quiet, electronically and mechanically, built like a tank, able to drive any load known to man, and with damping factor enough to tame even the most bloated sub system.

For the money, I would seriously consider a used Classe CA-100. You can find this outstanding amp selling typically for around $550 - $700. In fact there are a few up on Audiogon for sale right now. They are available in black or silver.

I owned a CA-100 at the same time I owned a heavy Adcom 5-channel amp. The Adcom sounded compressed and thin compared to the Classe. In fairness, this was a multi-channel Adcom intended for home theater vs the 2-channel Classe. I've owned a number of Classe amps over the years (DR10, 10, CA-100, CA-200, CA-300) and they never disappoint. And for the money, you'd be hard pressed to find better, IMHO.

used B&K EX442 can be had for ~$400-$500 used. It smokes the Adcom...

As I've commented before in these pages, the audiophile community tends to disparage some of the entry level, yet high-value products, available from various manufacturers. Adcom belongs to this "Rodney Dangerfield" group -- it tends not to get much respect. However, you will find that many of the regulars who contribute to this forum have owned Adcom preamps and power amps at one time or another -- myself included.

Unlike some of the other contributors, I have also been a retail salesman for Adcom equipment. Adcom has had some QC problems in recent years, which began after they shifted their manufacturing from one of the high-quality assemblers in Taiwan to a facility in mainland China. Having said that, I'd also say that Adcom seems to have addressed most of those problems, and continues to do a a pretty decent job of continuing to design and sell solid products for fair prices.

Based on my experiences with Adcom amps, which involves a LOT of listening hours with various speakers, I'd say that a used 5500 power amp is a hell of a value. For around $650-750, there are very few amps that provide 200 wpc of clean signal, and will handle speaker loads as low as 4 ohms with aplomb.

I am less enthused about the 5802, not because it's performance is poor (it's performance is actually quite good), but because it has a lot of competition at its price point. As Sean noted above, the 5802 is a better amp than its predecessor, the 5800. It is still, however, a very large amp and it produces a lot of heat for a solid state amp. It thus requires a lot of space with good ventilation.

I have both bought and sold Adcom power amps on Audiogon and E-bay, and had good experiences in all instances except one (a dealer that slightly misrepresented the exterior condition of the amp, tho' it worked fine). If you make a point of talking -- by phone -- with the seller, you can usually get a better idea about the fairness of the deal. You should also check seller ratings, since a lot of good feedback is usually a sound indicator of the seller's honesty. Also, Audiogon has "verified members", whose addresses and credit have been verified.