The buyer of an ISP will look at the expectations that the seller has set with his customers to determine whether or not the customer base fits his programs and services. ISPs don't exist without equipment. When one is buying an ISP one is only buying the net value of the income stream that the ISP owns. The strenght and peculiar character of that income stream is the issue of value. The buyer's perception of his ability to make the seller's assets fit without churn is the key factor in determining value of the user base.
Sellers can also affect the market and establish value.
If there is not adequate reason to sell, hold. I love conversations about valuations. I am not an expert, but these are my thoughts.