Selling a hosting business is very different from "selling" customers.
In Portugal, you can't sell your position in a contrat without the other party's consent. I could be wrong, but I don't believe that could be possible in the USA. You can however sell your participation in your company.
The domains must point to the buyer's dns.
A potencial buyer would be interested in number of clients, hosting plans of each individual client including price, resources used by each individual customer, contrat time period, subscription date of each individual customer. I guess he would also be curious about your motive for selling some customers, but not all.