That will only work the first time they do it, and then the flippers would know.
I would suggest a bib requirement combined with pre-ordering and a max purchase amount (maybe max 2 of any one item, and a total max of [10] items). That way, the flippers won't purchase the bib since they won't be able to buy enough items to make the payoff worth it. At the expo, everything will still be a free-for-all with no restrictions, so Disney can still sell everything (including to flippers), but at least everyone with a bib could pre-order what they wanted.