As for private Equity, it probably depends on the fund, but I would assume most of them look for growth companies.
So then the question is who is left? Maybe another entrepreneur looking to diversify, then maybe you could try at one of those market places where you can advertise your website. However I wonder if you are not to big.
Evaluation depends on where you are located (US, Europe, etc..), target market size, competition, metrics (growth, churn, potential), Software as a service vs services/consulting, etc... If you are not selling a product (Saas), or you are not growing (which seems to be your case), be prepared to be valued at a multiple of your earnings, and not total revenue. As for growth, everyone will look at past growth to extrapolate future growth. If you have no past growth, no one will believe you that you will grow your business next year 50% more.
As for the fees, I would expect you to pay a fee when you bail out because the price is too low, or decide not to sell, as they have also done work on their side. Otherwise no one wants to work with you anymore as well in the future. Still you should increase their revenue share the higher they sell. Don't expect though that they know all the companies to target, or have all the connections to those companies.
Most buyers will also require you to stay onboard for 2-3 years.
You should provide more details about region, type of business, market, growth rate, then you can get better replies.