As Gomo said at the beginning, like anything it's only worth as much as someone is willing to pay. My guess would be it's worth a few thousand at best but I've never bought any work. If it is just basically a list of customers and they don't take a liking to the buyer for whatever reason, the customer(s) could go elsewhere, and they have effectively bought nothing.
That's the risk you take.....someone WILL buy it..
you might lose 5 maybe 10 but that’s about it