"If you do not know how to properly examine a stamp in question...get a certificate"
The problem I typically see when examining collections is that the collector thinks they know how to examine those tough/valuable stamps but don't. The other problem is that they tend to place way too much trust in sellers "getting things right" (especially when they're buying on
ebay or BidStart.)
I've looked at my share of collections over the years, just like Bill W., and know that many people feel exactly the way stampmaster does. They can "do it themselves" and see no reason to pay someone else to confirm what they already "know."
However, in a "surprisingly high" percentage of those cases, if I look at their 5 most valuable (uncertified) stamps, at least one is problematic...and when I say "problematic," I mean problematic to the extent that they "lost" more money on that one stamp than they would have spent certifying all 5 of those higher value items... They thought they knew how to detect expert alterations, weed out the deceptive fakes, and make those tough identifications. They were wrong.
In theory, stampmaster is right. If you're one of "the" experts in an area, there's no reason to get a cert (unless you ever plan to sell, or your heirs will ever sell - then certs on valuable items are "no-brainers" & worth every penny.) The bigger problem lies in people accurately assessing their own abilities (and more importantly, their own weaknesses.)