There is an art to doing these deals.
I avoid telling the seller that I don't agree with his valuation. That only serves to put him on the defensive and reduces the chances of finding common ground. You're insulting his little baby. Why would he feel motivated to do business with you?
With large M&A deals it's one group of lawyers and accountants battling it out with another group of lawyers and accountants. With the small transactions it's very much a person-to-person thing. And people do business with people they like.
Too often buyers go in believing that they are somehow superior - they confuse their position with that of a consumer in Tescos. Some buyers go in with a "I'm the customer and the customer is always right" approach and they almost always fail to seal the deal.
A better approach is to start by accepting the seller's valuation... but managing the process so that you end up paying only what you feel the business is worth.
And there's a knack to squaring that circle. That's where the professional smarts come in. Here again amateur buyers make grave mistakes. They try to reduce the price by finding fault/s with the business and use these faults (or perceived faults) to discredit the seller's valuation. And that's the worst way to do it!
There isn't. And, to be honest, if you're asking that question in the first place IMO you're top of the list of people who need to get professional advice* before even starting negotiations like these.
*Just for the record: I'm not talking up my importance in order to get some business. While I do advise on these deals, I'm fully booked up at present and not taking on new clients for the time being.