Quoted from Mogg:agree, there are definitely some brands and places to avoid. A lot of the compressors like Dewalt are made by rand ingersol (I think), so you may want to research who manufactures what. I would Be careful on craigslist, you wont be able to tell often if a tank is rusted out or badly maintained. that said, you could find a great deal there.
Yes, be careful. Got a no brainer deal ($30) on an older speedaire 20 gal horizontal with single stage twin 2-3/4" bore cast pump. Completely restored the pump but the tank was a little more rusted inside than I bet my life on. Ended up having to buy a brand new asme coded 20 gallon tank, new flywheel and all the other little bits (rebuild kit etc). For the price of this restored 20 gallon setup i could have bought a new porter cable 20 gallon cast iron pump setup. Nice thing is I get 5.7 CFM at 90psi out of this little bad boy. Just enough I think to run HLVP for playfields.