Slow winds that build up are never really the ones you need to worry about during outdoor Bicycle Bazaars. It's the sudden bursts of really fast wind that throw your stuff all over the place that you need to be worried about.
Also, I found out that the last outdoor Bicycle Bazaar was a Tory propoganda day. That pissed me off. So I celetaped some Green Party leaflets to the front of my table in protest.
Yeh, but I actually want people to be able to pick and read my comics. No one'll bother looking and buying if it takes minutes to get a comic off the table and tears some of the paper.
I actually use string. Four lines of it running across the table. Two protecting the comics, and two protecting the t-shirts. That's usually enough to stop the wind doing too much damage to the display, but there are still those sudden gusts that you just can't protect against.
If something doesn't move, and it should, use WD40. If it does move, but it shouldn't, use duct tape.