You are only able to edit a post for a certain amount of time after posting.
For the first post within a trade topic this is usually 3 days, for normal posts it is 30 minutes.
By editing the first post of a trade topic you can reset this timer, for normal posts this does not work.
The time in which you can edit posts can be extended by customer perks and verified user perks:
https://forums.d2jsp.org/verify.php