Suggestion to conventions: increase the cost of your dealer table rent by $100 and include one membership, but say, "$100 discount if you already have a membership." (Insert a different dollar value in the slot depending on how much you charge for memberships.) This gives memberships to the dealers who don't buy memberships in your convention until they decide to actually sell at the con, while not punishing those people who bought memberships earlier. This is particularly important for conventions like Worldcon and Westercon that generally don't set their dealer table prices until less than a year out.
Besides, people seem to like "discounts" and dislike "surcharges" even when the net cost effect is the same either way.