You should be concerned about that person because there is very little chance that someone sitting on 360 copies of a set, which likely cost somewhere close to $40k, is going to be pricing them higher than a small time reseller like yourself.
One of the things that has happened over the last few years to the secondary Lego market is that the number of larger resellers has grown significantly. As a result, there is more price competition than there ever has been before. When you have multiple resellers sitting on 100+ copies of a single set that retails for over $100 and is rarely discounted, that is a recipe for pretty fierce price competition that will depress the price of the set until those resellers begin selling through their inventory. I could be wrong and this could be the one guy who has pie-in-the-sky dreams of selling all 360 copies of the Camper for $500+, but I think that someone who is willing to sink $40k into a single set is going to be far more pragmatic.
What you should be hoping for is that the demand for the Camper van is so off the charts that he'll sell through that stock in 6 months.