I have a Whelen Responder that has Val u bar end tags so I presume it would use the same guts. The rotators do use a specialty bulb. Although you cant run to the autoparts store to buy them, I thought they were readily available online (same bulb from many other bars (my Edge, Guardian, Centurion (including arrow/alleys)) use that same bulb).
Imo, its difficult for a rotator bar to be worth $150. People ask $400 for a bar worth $75 so avoiding the impulse buy until you can look around a bit.
What condition are the reflectors in? (Shiny or dull, chipped or burned?)
I would offer him $100 if it is in good condition. Not a lot of parts to fail on it, just pre buy a bulb or 2. I probably have a couple spare (used) ones fairly cheap if ya need.
It seems like a lot of halogen bars on Craigslist or Facebook have not been selling very good for a while. If ya make an offer that gets turned down, revisit it later to see if they changed their mind