It depends on the context they have told you that.
It is possible, but does depend on the terms of your private purchase contract. If it does not state you can sell this on then you will need permission from the developers.
Also any profits have to be taxed. It is not impossible but is a lot harder and the reson is no one is keen to buy off plan any more mainly due to the illegality problems recently highlighted but also as prices have come down, the stories agents told of buying off plan and selling on making a profit no longer exists, and hasnt done for a long time despite what you have been told!
Anyway, I digress, the contract can be sold on, but most who do are making a loss on their investment. Yes developers themselves arent selling either but that wont affect it
Any buyers of off plans I have had have agreed to take on the purchase but it is done at notary on completion date so they get a mortgage to cover the purchase as opposed toputting cash down months in advance.
If you want to give more info - by PM if you prefer, then I could possible help you further