I never even look at whether one has a DNF Seal or not.
However, I will look at trader feedback, if one has any, and also sometimes look at their past posting history / google, etc.
Problem with relying on the DNF Seal is that a scammer can easily obtain one - either directly by applying for a DNF Seal ... or indirectly by hacking a dormant user account, etc.
On a related note, in light of DNF being hacked, one can't be certain that everyone who has a seal truly obtained their seal legitimately.
And finally, there's nothing that precludes one copying the DNF Seal graphic and uploading it as their avatar - many people would be fooled unless they mouse over the avatar graphic.
Ron