Comic dealers have been doing this since the very start. In fact, there is now a "Golden Hour" if you will where an hour before the doors open dealers are free to make deals with others and scope out the competition. It is however bad form to bother anyone during set up. Once the announcer says it's OK, you will see a significant amount of action till the doors are officially open.
I worked many a show during the late 80's early 90's and the only one that didn't have it for years and then finally adopted it was Big Apple Con.
Hobby vendors need to adopt this.