It's hard to say value. People don't pay a premium for them unless they know what they are. The age and speakers make a difference. Get the age off a speaker code, and get the T# to get the model if you have the Marshall logo backs. I paid $375 for my A and I have the G12H80's in it (T2871), from Jan. 1980. 320 watts of capacity with the 120 watt sticker! LOL They usually sell for about the same price as a 1960 cabinet of the same vintage; mine was a little road worn but not too bad. I think I saved $100 over what it "should" sell for.
i have the 1982 a and b cab with small circle sticker reading "120watt" if you have any idea the value of this stack, it would be greatly appreciated. thank you