I have an opportunity to buy a used jet bd-920n lathe. I have used the machine before and it seems to work fine. I'm wondering what I should offer as a fair price for one with out the stand.

I see they are going new for $1900 or so, I can't afford that but I don't want to low ball the guy.

I've owned that same lathe years ago. I bought mine from a machine shop/tool dealer in Modesto Ca. (Rayco Industrial) a few cities over. I beat the crap out of it while learning. It ran almost 3-4 hours a day for almost nine years, of course replacing belts, worn gears, and the power switch more than once. It was beat to hell when I finally took it to a local scrap yard and got scrap value of $20 if memory serves correct.  The only reason I would've sold that lathe was either to upgrade to a larger machine which I did or if I ever decided to hang it all up and not make metal chips ever again.

I would personally start low and work up to something you're comfortable with. The sale price will depend on condition of the machine and if there are any extra tools that go with it. The problem with that little machine was the small spindle bore hole which is right about 7/8" and the relatively short length of work (20").  My advice is to buy the biggest lathe possible within your budget. I ended buying a Grizzly Industrial 11X26 lathe with a 1" spindle bore. I have used the Grizzly for almost five years now almost on a daily basis. I should have gone with a larger lathe but I did not have the space and funds when I bought it.

You may think the JET is everything you will ever need until you start working more and more with metal and realize the 9X20 is really a starter lathe. I would offer $500-$600 and not go over $700. The seller may get offended but they may be ready to upgrade and need some cash and have the machine out of their hands before buying something bigger. My Grizzly is going for $1,555. I would buy that over the JET any day. The only advantage the JET had was it was easier to change gears for threading with a few shifts of the knob.

I usually thread 16, 20, 24, and 28 TPI and I keep the gears in the same spot and gear changes only takes no more than five minutes to switch over. You will hear people tell you to buy an old American made lathe but the problem you may have is the lathe may not work correctly or may need repairs/upgrades to get it back in working condition. Hope this helps.

To answer your question directly, not more than $900.

Indirectly, this is not a lathe for barrel work. You want a bigger hole through the spindle, and you want more power.
When faced with this (last year) I bought a 12×36 lathe weighing in at 1200+ pounds with 2HP and 1.75" spindle bore.
You will also want 16mm (0.6") lathe tooling preferably with replaceable carbide inserts.

I went with the 12×36 with a 1.57 spindle bore for barrel work.

To OP: Buy the biggest heaviest lathe you can afford and have room to fit.
Tip #2: save some money as you will need it for tooling and raw stock.

Just sold mine for 800.

I thought I did well