A buddy of mine that is closing up shop has his sold for $2,000 and the buyer has to take it down and move it. For $1,800 installed, it would seem to be a good buy if the lift is in good condition.
If it's well maintained I think that's a fair price. I paid $900 for my 7K Rotary two post but I had to pick it up and install it myself. It was already taken down though.
Take a close look at all the lines, make sure there aren't any leaks, also you may want to look closely at the piston, a common issue is a leak from the seal. If the cables are in good shape you should be fine.