Probably going for a discounted or heavily discounted price during the ammo shortage.
Have to probably find a reloader or a collector willing to wait.
Saw one dealer out of most ammunition except ammo going for $3 a cartridge. Some rifles are getting expensive to shoot.
Because of the caliber, and assuming it is as advertised, you should expect to get offers north of $750 if you were to list it. Anticipate low ballers, but the .257 Roberts has quite a cult of followers willing to add this gun--one that is not readily available--to their respective collections and will pay a premium to have it. You can always lower your asking price so I would start with a rather bullish price and go from there.
I also have A .257 Rbts in the Remington 7600 Pump Gun. I ended up reloading Sierra 100 gr. gamekings for it, very deadly on Deer. If You have a Buddy that reloads, that would be the best option. I have a few precious 117 gr. I save for Bear Season. As to selling Price I would not part with mine for less than $600.00 .