I have 2 Buster's. Both were the 15" combos- but I pulled one of the heads and ordered a head cabinet from Mesa (cost me over $200 for the Mesa cabinet).
...and a fun gut-shot!
Mine is in near-mint condition (the amplifier itself as well), but is due for a re-cap (there's a tiny amount of hiss when cranked up). I've been considering selling mine (since I have 2), but probably wont as I love it so much!

Even with the investment in the actual Mesa cabinetry, I wouldn't expect to get more than $700 for it. There just really isn't a lot of resale value in custom-made cases (maybe half or less what you pay).
I'd really want to see how the Buster in question was re-housed before dropping anywhere near a grand!