Placing a value on a used machine is difficult, especially without the opportunity to examine it closely first hand. To give any kind of opinions (and they would be just that, opinions, not an appraisal) some more information would be needed. Even then, proper evaluation is nearly impossible, because some things have to be felt or checked with an indicator, not just looked at.
Swing of lathe
Length of bed (distance between centers would suffice)
Detailed photos of the ways
Detailed list of accessories.
From the photo it appears that this lathe comes with a 3 jaw chuck, two 4 jaw chucks, a faceplate, a dog drive plate, an assortment of collets (maybe not a complete set), a collet chuck, a drill chuck, and a quick change tool post. I assume that it probably comes with some dead centers, drive dogs, and live centers as well, but you will need to look.
This lathe does have a quick change gear box, so yes, it is capable of threading. I cannot tell from the photo if it has a taper attachment or not.
There are plenty of accessories which will help with the value, but the real indicators will be the condition of the ways, condition of the backgears in the headstock, and the spindle and spindle bearings. The ways should be smooth and free of nicks and gouges. The gears should not have any teeth missing. The bearings should all be smooth and quiet, and not allow and side play or end play in the spindle. The spindle should be free of nicks and gouges on the inside Morse Taper, and the chuck mounting threads should be clean a free of damage.
In addition, without cutting a test bar, it is impossible to tell how much wear there is on the ways. You should also check to see how much backlash there is in the cross slide and compound rest.
This lathe with all of its accessories might well be worth the $1800 that the seller is asking, especially if everything is in good shape and does not show excessive wear, but don't take my word for it, as I have not been able to give it a thorough examination. It might be a good idea to see if there is someone in your area willing to go with you to evaluate it.
If you choose to purchase this lathe, and it is in good shape, you will not be disappointed with it.