John is indeed correct.......when I'm in the market for a particular set, the COA is a must have for me. Now....... who resurrected this old dog of a thread?
I have a plethora of US proof and mint sets and created posts on the mintage and value of the clad, silver, and UNC sets. The clad proof sets are really not pricing well in the market with the possible exception of 2012 and anything before 1964 (silver value). If the set doesn't have OGP including the COA or they are damaged, the sets are even bigger dogs. Unless you really want the sets, I would pass on the purchase unless the prices are very very low.
One thing to remember for people insisting their sets have COA's, the mint did not start using COA's until 1986, and the early ones did not specify what set or item they were certifying as genuine.