Get a confirmation from the buyer as to which bank so you can check the reference number and inform them the deposit has not appeared in your account. If no response within the next day or so, I would open an unpaid item case.

However normally a DD doesn't take long whether they have walked into a bank or over the internet.

Often it appears within hours of depositing. Sounds strange.

In the meantime contact the potential SCO recipient and explain the buyer still has not completed purchase and you may be sending a SCO if they are still interested.