If it doesn't have any cosmetic issues, and it works fine, that's a fair price. No bargain by any means, but a fair price.
If it doesn't work fine, all inputs, AUX and Phono especially, no hum, then it's too much.
If it has cosmetic issues, i.e. the front panel has flaws or missing letters, or the chassis has rust or corrosion spots, or is missing lettering, then it's too much.
If it doesn't work fine, and it has cosmetic issues, then it's way too much.
If any of the trannys, either the power or output transformers are bad, then it's waaaaaaay too much. In that case it's just a parts unit. $50, if it's got all the knobs. Bad trannys and no knobs, it's not worth shipping cost. Buying used transformers can cost you more than you paid for the amp if they're bad.
Sovtek or JJ 6BQ5/EL84 outputs are less than $10 each from Antique Electronic Supply and most outlets. 12AX7's are in the same ballpark, so replacement tube costs are not the issue. There are folks who will pay much more than that for some NOS tubes. (I have 8 NOS black plate RCA 6BQ5's in the mint original boxes that I stole for less than $80 total.) But, you truly don't need to get into that in the beginning.
Believe me, the amp will sound very nice indeed with whatever outputs you feed it, as long as they're working correctly.
Good luck with your negotiating! :boxing:
Clay