Similar to real-estate to a certain degree. A lot of it is based upon prior experience and having sold like businesses usually. Many also belong to trade groups or similar organizations that provide deal information that they can look up anonymized deals and find like values etc.

It is hard to hack your way through this one if you want accurate information to maximize your valuation.

I'd also encourage the potential buyer to value the business first, with providing only some basic data points, and that way you know if there is even a real opportunity for both parties. Cause if you are thinking you are getting some multiple of revenue (or net) and they are saying the book of business is worth $50k, you could be off by an insurmountable number and better to know that earlier then later.