In most peoples advice you will get the replies that you should never buy a fish which you cannot properly house for its adult life. I mean lets be serious..very few people buy that "future" tank they plan on getting.
I'd say that 75% or more new aquarium hobbiest who go for things like sharks and aggressives even stay in the hobby after a couple years.
And the whole idea of buying a fish, keeping it for two years then finding it a home is such a poor plan of action. Very, VERY few aquarium keepers can/have a proper set up for the larger demand fish. While its not much trouble/expense to buy a 75 gallon once you get into the 300+ gallon range it start to get expensive.
The idea of giving a 2 foot shark back to a pet store in hopes of it finding a home is also a bad plan. Even if the store takes it back chances are its doomed.
My advice... Do not buy things you cannot properly take care of for the full extent of its life. The shark for sale in whatever pet store you are looking at is doomed. Plain and Simple. But since the pet store owner bought it in the first place he or she is all about the money and doesn't give a damn about the animal's wellbeing. If that shark doesn't sell, the owner will be less likely to get another, and it will save a shark from being collected in the long run.
So, you can know your shark is swimming happily in the ocean somewhere!!