The shop is looking great! Did the maple syrup boiler thing go to someone who will use it? I don't think you will see much difference between a 6gal vs 8gal. The cfm is what is important. If you find you need more capacity you can get a broken compressor, often for free, and salvage the tank from it. I would keep your compressor as is and plumb it to the second tank. You might not need a second tank though.
It's the principle of the sale....false advertising and all that. If it had been advertised as a 6 gallon, I would not have considered it. Imagine buying a pick up advertised with a V-8 but then only getting a 6 cylinder........there would be a price adjustment, no?
Getting a $50 "store credit" as a price adjustment wouldn't do much for me, as I wouldn't be likely to buy from these folks again if they're that misleading. $50 cash, to me, would be different. But if you're likely to actually use the credit, then I guess all is well.
This is an online business that sells and delivers just about anything. The wrong tank size of the tank was given by the manufacturer-not the seller. I will use them again; probably less than a week from now. But you're right.........if it had been a local business....it would have been returned.