sharksfan, To have a stamp expertised means that the stamp is real/genuine, not forged or otherwised tampered with, very important with overprinted stamps can include grading, cost is usually a percentage of the estimated value.
It could become very expendsive. An expert will mark your stamp with their ID & if found real and a valuable issue it will increase your return on the investment.
I would recomend to check the APS site:
www.stamps.orgMike
PS Did you have something in mind to check? Let's see it!