I surely hope the truly expert will respond, but until then, I will rush in.
Before you go, do a Google on "Rockwell Lathe", and "Rockwell Lathe Practical Machinist". What pops up? Are the responses of the "I need.......", "Does anyone have...." variety? Not a good sign. When someone does ask, are there responses to the post, or is it simply 'crickets chirping'?
Run a thumbnail/fingernail up and over the ways within the first foot or so of the headstock. Does your nail hang up on any ridges? If Yes, I don't think the lathe is worth the asking price. Others may disagree, but I want to give you very conservative advice.
Are the ways dry, or is there a slight film of lubricant on them? He knows you were coming, seems to me a slight film should be present, even if he put it on as you rang the doorbell.
Has anyone's resting hand worn a spot on the headstock?
Take a look at the gear train, gearbox, etc. as possible.
Have the seller go through the "all speeds, all feeds" routine. What do you hear?
Take a look at the carriage, the chuck, the toolpost etc. and look for signs of a crash. "A new chuck" isn't always a good sign.... Chunks removed and gouges aren't signs of normal wear.
Take a look at the tailstock. Feel inside the taper socket, do you feel any wear rings in there? If yes, someone was spinning a drill bit in the taper socket.
If you can, check the taper on the spindle bore. Check for signs of spun tooling.
How big of a spindle bore do you desire?
Do you ever think you will be cutting pipe threads? If Yes, brush up the TPI of potential pipe sizes, and look for them on the chart.
Is the threading dial missing? That's a real easy $100 to replace, perhaps more than $200....
Crank the tailstock in and out. Does it?
How smooth are the various handwheels? How much play, CW to CCW?
What is bent? What is missing?
Check the lubricant levels, if sight glasses are present.
The 14x41 Rockwell's asking price seems to be $2750 or so. $1800 for that (smaller) unit doesn't seem too bad.
In the future, set up and Save This Search on ebay, under My Ebay. You can get email alerts whenever your search terms are found. Do that for a year or so, and you will have good idea on how often the item comes up, and the typical pricing, and the typical geographic location.
Okay, that's enough from me, others can contribute.
Steve