I have the Sigma 150-500 OS lens ... yes it has IS (Image Stabilization), Sigma calls it Optical Stabilization (OS).
I bought brand new for around $940 ... I also used EF 100-400 L IS for over 3 years.
For under $1000, Sigma 150-500 is worth its price ... it is not as sharp as Canon EF 100-400 L IS (which costs almost $500 more and the reach is only up to 400mm) but close enough.
I don't know what "A/B" condition means for that used lens... but for a little over $100 more you should be able to buy a brand new Sigma 15-500 OS ... look around for deals.
If that used lens is in very good condition, and keeping the current market price (for a brand new one) in mind, I would say under $700 is a reasonable price to pay for it.