Actually, his prices aren't bad. That's actually better than current going prices if this stuff works.
The motor bases usually go for $25-$35 or so. The drums usually go for $20 or more, especially with reels. The inversion tanks would probably easily fetch $10 and up, more for the taller ones.
If you're lucky you can snag these cheaper, but not often. And you'd probably be waiting a while to get lucky. If you think you can assemble this much stuff for $135 or less, good for you, but it seems fair to me.
OP, if I needed all this stuff, I'd buy it in a second. If you're interested in piecing it out, I may be interested in a thing or two. PM me if that's the case. Otherwise, best of luck. You've got a nifty little set up there.