PC DOC's comments are right on. The biggest problem with a "one man band" service type business is that the "one man" IS the business. I don't think you'll get much more than what the current value of the assets.
When this situation comes up in other service oriented businesses, the seller will often stay on smoothing over the transition and training the new owner. It makes the situation closer to selling a "business" rather than a simple transfer of assets. This type of arrangement can add significant value to the potential deal.