Good advice from our colleague from Arizona regarding gussying up your machine as much as possible, and taking excellent photos. These days, folks who make videos of their bikes being started and run, and post them on U-tube, with a link from their ad(s), can generally expect the time spent to be worth the effort.
But, I think he might be a bit optimistic on asking price. It's true that prices of 2nd hand RE Singles increased noticeably when Royal Enfield announced the discontinuation of the 500s. But with the arrival of the new OHC 350 Classic at US dealerships just a few weeks ago, I think the market has subtly shifted again. I think that these machines, which can be purchased brand new with a warrantee for as little as $4500 plus dealer fees and charges, will attract a lot of the "dream-cycle" market, which will put downward pressure on the prices of similar used machines.
Although members here in the forum can easily tell the difference between a old Iron Belly Bullet and one with a newer engine, I personally found that my UCE Classic 500 attracted more "nostalgia interest" than my older machine with IB pre-unit construction. People were responding to the old timey styling, which is intentionally more blatant on the Classic, than it is on the Bullet, which is only incidentally retro. And, nobody wanted to tell stories about how they had a Royal Enfield back in the day, it was always about the BSA or Triumph that was in their life back in the golden days. The RE aroused an abstract nostalgia about classic motorcycles, but never anything specific to the make or model.
Judging from what our own members have said here in the forum, those who are searching specifically for IB machines, and rejecting UCE candidates, are doing so because they want the mechanical challenge required by the old technology. I doubt that those who want a bike to ride on warm summer evenings, or to put on display, will pay a premium to acquire the more scarce IB.
But, you never know, and it only takes one buyer to make a sale.
I'd look at the comparables and price accordingly. You can find some here:
https://www.2040-motos.com/Royal-Enfield/https://royal-enfields.blogspot.com/https://www.classic.com/m/royal-enfield/https://www.cycletrader.com/Royal-Enfield/motorcycles-for-sale?type=Motorcycle%7C356953&make=Royal%20Enfield%7C9301341