I own two circa '70-'71 ("silverface") 150s and I paid $50 for each of them in Craigslist deals spanning one or two years. I got really lucky both times, but typical eBay completed price has been around $150, $200 tops for a long time. They are really excellent heads for both guitar and bass. The 120 has the EQ which should make it better for bass in theory, but it really shouldn't make any difference.
My general rule of thumb is that a seller can / should expect less in a local sale than they would on eBay. (They might actually end up getting *more* in the end after all the fees and perpetual eBay shipping calculator fail.) Maybe bring your local guy a few printed-out completed auctions and see if you can leverage reality to your advantage.
(edit: also keep in mind that none of these are super powerful, although ACC tended to rate things based on "honest" RMS. The 120 / 140 / 150 are all rated roughly 125w into 4 ohms. That said, they sure seem a lot louder than that, and they are definitely pretty bulletproof.)
Last edited by lossfizzle : 07-25-2013 at 05:09 PM.