I'm pretty sure that to sell stuff you have to rent a table. Probably not worth it for a lens. You could set up a pirate table outside on the street, that could be fun. Generally I think the sellers are dealers and collectors trying to offload some stuff. For one lens you might be better off trying to list it on APUG, ebay or in the Trader. Or perhaps take it with you on the day & mention to the tables selling Leica gear that you have it. They might do a deal or fix you up with someone. In that case make sure you know in advance what it's worth.
Finally, anyone have any experience in selling items there? I have a LTM collapsible Summicron in the 125XXXX range that I am thinking of selling, wondering if it is worth taking along.