At the recent ASDA Stamp Show in Fort Lauderdale, a collector was recommended to come see me re a small collection that he had inherited. He told me that it was not worth much but that it contained a couple of Ceylon, Queen Victoria stamps that maybe valuable. One, he told me, had been expertized but he had misplaced the certificate. However when he 1st received the certificate, he had offered the stamp on eBay but had not received any bids.
The alleged expertized stamp was thought to be the 1857-59 one shilling violet Scott # 11. It was in fact a one shilling purple, Scott # 57 (issued about a decade later) that had had the perfs trimmed so that it appeared as the imperforate #11. While it would easily be noted by many of us, it was sufficiently like the #11 to fool a novice collector.
It was not easy for me to have to tell the visitor to the show that what he had was worthless, but in fact he appeared grateful for the information that he received.
There is a charge to employ any expertising company and often it is worth the expense. However I recommend that if you have a potentially valuable stamp or cover, ask other members of your stamp club, or take the item along to a stamp show & ask a dealer for an opinion (ensure that he is not too busy before approaching). If you get a positive reaction, then, by all means, send on to an expert.