I have had a cash deposit reversed, a buyer paid over the counter at a branch, then reversed the payment straight away, then paid again... I presume they reversed the payment because the first one had no reference, and the second one had their eBay ID as a reference, but it proves that bank deposit (at least at a branch) is reversable... 

I personally don't get the opportunity to cancel an online transfer, though, which is how they would have to do it when out and about. It's possible I could contact my bank and say the transfer was unauthorised, but I don't know if they would do anything about that because to perform a transfer, it requires login info and an authorisation code sent to my mobile, (unless a previously saved / authorised destination), so a bit hard to argue "unauthorised". 

Probably raises more questions, or at least doesn't really answer yours, but that's all I really know from experience.

Edited to add: didn't think about transfer clearance time as the others have mentioned, makes it very much less than ideal to accept at point of sale.