I am assuming you have done your research, are handy around engines, know that almost all of these are going to need some work if bought at auction, etc. So I am going to get right to the point on auction sites in no particular order:
1, Ebay, steady supply, lots of semi-professional resellers, some sell as is, others do some refurb, some of the refurbs are of the rattle can variety. Every once in a while there may be one that sells cheap, so it is worth looking, most are sold local pickup only
2, GL (Government Liquidation), these are almost all going to be DRMO turn in units from the military, they are the ultimate AS-IS sale, and dealing with GL has about all the pleasantness of dealing the the DMV, this is not to say that there are not pleasant individuals some of the time, but as a system it can be a pain. Over the years I have bought various equipment from them (6 or 7 generators, a couple of fork lifts, diesel air compressor, water pump, jacks, and even a file cabinet, and probably half a dozen things I am forgetting....) Except for a couple of the hydraulic jacks, and a couple of things bought as parts units, I have been able to get it all running, with only moderate amount of work, I might be very lucky though, and certainly pay attention to details. As others have said prices are up, a lot of that is on the east coast though, and GL does not ship, they do allow pick up by some freight companies though, well anyone, but many freight companies will not want to deal with them and their pick up appointments and potential 2-3+ hour wait once you get there, hassle of getting on base in some locations, etc. Condition can vary all over the place from dead field turn in units, to fresh rebuilds and everything in between. Out of generators I have bought from them only one started ran and generated power with only fluids and batteries, and it quickly developed a problem with its battery charging system (needed new $100 voltage regulator). Since writing the earlier replies in this thread I have bought 3 more generators from GL for myself, and 1 that was part of a batch I passed along as it to another forum member. A MEP-016D yanmar conversion, a MEP-002a and a MEP-003a, all 3 were relatively easy fixes, the MEP-016D (paid a bit too much at $450 thanks to a dead low hour meter, someone had installed a DC hour meter on an AC circuit) just had a clogged fuel filter that needed attention to get it running, and a few secondary semi cosmetic things, like broken wires going to the panel meters, and stuck belly tank fuel diverter valve, stuck in external supply position. MEP-002a bought for $400 or so last summer seems to be a fresh from major service (but not full rebuild) with all new filters, new starter and new fuel tank installed, ran first starting attempt, but voltage was high (154VAC instead of 120VAC), had blown $3 transistor on the voltage regulator (very common problem) 5 minutes with a soldering iron fixed it. MEP-003a bought a couple of months ago for $1,100 or so, needed only new filters, batteries and fluids, tank had some gunk in the bottom that had to be cleaned out, but not nearly as bad as most people report, to started. I call this one my stealth MEP, it is painted solid OD green, has none of the common stencils not even a DIESEL ONLY on the fuel tank, and is missing some of its data plates like the starting instructions. Unit markings suggest it belonged to a now disbanded Air National Guard rapid deployment force. Engine runs a bit rough until it warms up (still working on that), and it did not generate power when started. That one took some time to locate, but traced it to a voltage control transformer that was installed backwards. This mistake is somewhat understandable when you see the logical wiring schematic on the top of the unit shows the 4 wires in reverse of physical position. Still it is obvious from bits of marker tape, etc. that someone had tried to fix it by the book and gave up before locating the problem 3which was WAY down the test procedure list. Given this may likely have been the person that installed the transformer backwards, one has to wonder about their skill level.
3/4 GSA Auctions, and GovDeals These are both going to be auction sales of surplus material from government entities, many of which received hand me down generators from DRMO that did not go to the GL sales process. Think of all of these like you might a 2 owner used car vs a 1 owner. Again you can find pristine rebuilds here, they just likely have sat around longer, some will have been used, and modified / civilized with such additions as electrical connection boxes grafted on, or being painted fire service red, you can also find odd mix and match stuff, like a MEP-002a mounted on the type trailer you usually see a MEP-004a on. I have bid on a few, but never won anything from one. On the plus side some of these people can be a lot more flexible to work with than GL, sometimes will tell you history on the units, if trailers are/were road worthy, if tires still have air in them, etc.
Ike