IMHO if the pet store bought it as a het to sell as a het then they would know what it is. If they don't know what it is then it is just a normal. Even if it really is het there is som much work you will have to do to try and prove it. Whats the het percentage? It could be het for a new morph but if its only 5% het what are the odds you will ever pull that morph out of it?

Even if it is a het no one can prove it at the time of sale so I would only pay normal prices. But if you like the way it looks and are good with the possibility of never proving it out then buy it