If you are buying a site without accounts, in essence you are buying traffic.
If you are sure of where the traffic came from and that you can maintain the level of traffic, you can expect to eventually build up new accounts with this traffic.
As the Professor points out, it takes time to build up a player base. The price of a site without accounts is based on traffic numbers as I see it, and knowing exactly what type traffic you are buying is crucial. The previous income is a tool to help ascertain that, along with the source i.e. SE visitors, bought links etc etc.
The age of the site matters a lot because it will be an advantage in generating SE traffic.
Everyone seems to have their own formula as to how to value a site, this is mine for a site without accounts.
As far as the seller goes, be aware that programs with predatory clauses, such as confiscating earnings if you don’t deliver x players per period, will make holding on to accounts futile.