private company valuation U.S. GAAP SFAS 157

there is this subject about " the effect of value definitons on estimated value" ON PAGE 309FROM SCHWESER. i dont understand what they exactely mean by this sentence: Under US GAAP SFAS 157, fair value is the exit price (i.e. the price received by the seller), which will likely be lower than the entry price paid by the buyer in a transaction. So what does this exactely mean. can someone make an example…sorry i just don’t get it. in a transaction the price paid is one price. so what is the conclusion here? and other than that is it the RULE, that when using GPCM (the comparable price multiple data is from noncontrolling interest) and when using GTM (the comparable price multiple is for the sale of entire firms where control is acquired).?