The customer had recently purchased several expensive (to me) covers. He wanted the dealer's opinion as to their genuineness. One he had paid $600.00 USD for and the dealer told him he got it at a very good price. Another he said he paid more, but would not give a figure. He asked the dealer after being told they both looked genuine if he should have either or both expertized. The dealer said no, one of them had no record of ever being forged, the other looked good.
I do not know what your financial situation is, but to me a $600.00 cover should be expertized. $600.00 is a lot of money, and manipulating a cover is more common than most understand. The skills needed to change a $50.00 cover to a $500.00 cover are held by many. By far the majority of stamp collectors are honest and trustworthy, but that is exactly why buyers should beware. As we get older our knowledge base gets larger (that is good) but our eyesight and thinking often get numbed. The greatest skill a con artist has is getting the client to trust him/her and drop their guard.
Maybe I am a cheapskate, but I would not pay $600.00 for a cover unless it had a certificate, or the seller would allow me to have it expertized.
As a note I am one of the PRC expertizers for both the CSS and the APS, so perhaps I am prejudiced, but I worked hard for what money I have, and do not want anyone taking advantage of me. When I expertize a stamp or cover I often spend 20 minutes or more studying it, under natural lighting, long wave UV, short wave UV, watermark fluid, magnification to 20 times, and a number of other ways. That is quite a bit different from looking at a cover as I turn it over several times as I carry on a conversation.
If it is expensive to you, get it expertized!