what have I got?

I have no idea what the capacity or HP of this beast. It was used for nailing mostly. Can anyone tell me where I might be able to get information on it, what it's worth, etc? I'm thinking of selling it and have no idea of what it should go for.

By big tank I assume it is in the 20-30 gallon region, since I doubt MW would sell anything industrial sized, so here goes.
Sounds like you may have a Campbell Hausfeld VT series, since they were the most common type of cast iron pump available for home use. A dead giveaway is a two cylinder vertical single stage pump with six long bolts running through the head along the outside of the block, down into the crankcase which holds the whole shebang together. Also look for angular cooling fins \\// on each cylinder ( old old style ) vertical cooling fins on the cylinders and head ||||| (old style) or horizontal cooling fins on the block (later style). All pumps are good, but the second version with the vertical fins and the two piece heads are the best of the three in my opinion. If this is not the pump you have, I would have to see it to tell you what it is for sure. There are a bazillion different types of smaller compressor pumps. If it is a C-H, it would probably be a 1 to 2 hp machine. HP should be on a metal tag on the outside of the motor. Wouldn't be able to tell you what to sell it for without seeing it or knowing what similar sized machines are going for new in your area. Sorry.