I missed a plain restricted 59 at a local gun show last year priced at $500 with the B&P restricted barrel. A great deal at that price.
Your 5904 is a newer model and worth more, in my humble opinion. In mint condition, it would be a gift at $600. I'm sure Barry charged at least $300 to do the barrel. Frankly, I wouldn't sell such a thing for less than $750 if I had it in the condition you describe.
If the laws ever change, the original (factory) barrel can always be found and re-installed. A job of a few minutes at most, and there are lots of the prohibited barrels in the USA for under $100.
These are good shooters, though I prefer the single stack models (39, 639, 3904, 3906, 3913, etc.) and of course buy them in their 12(6) configuration. Minty S&W auto, normally prohibited, now restricted and available to all those guys who always wanted one? If people were smart, your inbox would be full of PMs already. They wouldn't wait to miss it on the EE.
It doesn't help that you have no feedback, though. I'd get busy and buy and sell some other stuff. Maybe minor things, but build up your feedback first.