For example:
we have 1 circlet blue unid is this the same as buying from gheed?

Depends on where you got the blue circlet and difficulty. It will factor into its item level where as gambling is based off your character level.

^ this.

From what I remember and when gambling, it's plus or minus 5 levels. If you are lvl 90, then your gambling window is lvl85 to lvl95.

if character 1 lvl use Scroll Indefinity on unid Circle and 80 lvl charachter ,
difference ?

No, the circlet will always be the same as it is based off the monster or chest that dropped it not the character identifying it. When gambling it will be based off the characters level.

Continuing, the circ stats/mods are already determined when it drops, it doesn't roll when it's identified

+6 and -3