Assuming you are referring to the listing by seller SkyFi, given the inclusion of a new set of tubes and the stated condition, as well as the seller’s extensive perfect feedback, the price is very fair. The shipping cost is also very fair, given the amp’s 100+ pound weight. If you were to wait for what would probably be a **long** time to find a unit in similar condition from a private seller who is not in the business of buying and selling audio equipment you might be able to get a somewhat lower price, but this seller’s price is certainly well within reason.
However, I would make a point of asking him if the new set of tubes was supplied by VAC, and if not what assurance there is that the 300Bs are suitable for use in this amp. 300Bs that are compliant with (or better than) the original Western Electric specifications for maximum current, voltage, and power are said to last pretty much forever in this amp (exaggerating just slightly). But those that are not compliant with those specs (and a number of currently produced 300Bs apparently are not) will suffer a very quick and probably spectacular demise. And could very well damage the amp when they fail. If the tubes were purchased from VAC, though, they should be fine.
Also, I would recommend replacing the Chinese 6SN7s with vintage tubes. Among many others I tried in my amp I had excellent results with 1950s and 1960s Sylvania and GE 6SN7GTBs, which can be had for reasonable prices. (Non-suffix vintage 6SN7s, btw, are NOT suitable for use in at least two of the four positions in this amp. I’m not sure about 6SN7GTAs).
One reason I make that suggestion is that although I’m pretty certain the tube set in my amp was purchased from VAC by the previous owner just prior to my acquisition of it, one of the Chinese 6SN7 tubes it came with abruptly developed an internal short after about two hours of use, suddenly resulting in a blood-curdling noise from one of the speakers. Fortunately I was in the room and had the amp shut down within about four seconds.
The Chinese 300B-98s that came with my amp, though, which I believe were purchased from VAC as I said, worked great throughout the seven years I owned the amp.
Good luck. Regards,