An eCheque is a Paypal payment funded through a non-linked bank account. It takes as long to clear as a paper cheque.

The same rules apply. If you don't have the funds in your account it will bounce and you will, hopefully, get hit with a bank fee. if it does bounce you will also have a less than happy seller.

Why would you buy something if you don't have the funds to pay for it? It's not a game, you know.