What In This Stamp Title

what is the meaning of title of this stamp I epay.
https://www.ebay.com/itm/MOMEN-US-S...40M&LH_BIN=1

Which bit? Momen is the dealer. PSE certifies stamps with grades based on both their condition and their centring - this is a system only used in the US, as far as I know. Gem 100 indicates that PSE thinks the stamp is perfect by its terms of reference.

why that price I saw it is common

It is graded 100. That is perfect centering. To some collectors that is a big deal.

Some people, including myself, believe that grading is a scheme to make common, low value stamps seem more desirable.

As the song says, "nice work if you can get it".

Not only grade 100 but a jumbo at that. Don't think I've ever seen a 100J grade before. While a grade 100J stamp should garner a higher price than a normal grade 80 (VF) stamp, the values stated for stamps above grade 90 or 95 seem rather ridiculous, especially for common stamps. While such stamps tend to be very scarce, the difference between 95, 98 & 100 is, in my opinion, too small to be worth the astronomical values assigned them by the philatelic powers-that-be. Of course, if one has more money than they know what to do with, they can choose to collect only very high grade stamps. VF is easily good enough for most collectors.

The J after the 100, indicates that it has Jumbo, or extra large, margins.

... the D before the umbo describes the prospective purchaser ...

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T before rumping your D before umbo ...

Cheers,

/s/ ikeyPikey (with apologies to the OP)

Ikeypikey, No comment other than -