Under what circumstances can a bank transfer be reversed?

You can't reverse or cancel a direct bank transfer (sometimes known as BACS or Faster Payments).

These scams typically work in one of two ways:

• The scammer uses a reversible payment method, for example giving you a cheque which may appear as cleared in your bank account but may subsequently be detected to be fraudulent many months later and then the money removed again
• The scammer makes you a direct transfer from someone else's bank account (either an account he has directly hacked, or more likely he convinces another of his victims to make a payment to his "beloved Grandmother who is stuck on an oil rig" or whatever). You get the money and think it's legit and send the item he is buying from you. A few days later the victim realises their money has gone and contacts their bank. At that point the fraud department of the bank may work with the fraud department of your bank to return the money that you got unlawfully from the victim. And if you're doubly unlucky then the police don't believe you that there was a third party involved and you are up on charges of money laundering and/or fraud yourself.

Bank transfers are reversed in specific cases of fraud.

For example, if I hack into your bank account and send money to a third person, when it is discovered it will be reversed. There is no artificial deadline for that to happen.

On the other hand, if I trick you and convince you to send money to that third person, that cannot be reversed. The difference is that it was you who told the bank where to send your own money, and you can’t just change your mind later.

If you are a seller, the best way to protect yourself is to ensure to the best of your ability that your customer is who they say they are. That is why in-person transactions often require photo ID, and why online transactions often will only ship to the address that is associated with the bank card.

A retailer could be the target of a "chargeback", this can occur legitimately to protect the consumers rights but can also occur in a more dubious manner where a consumer is attempting to bypass a retailers return policies.

How does chargeback work?

Chargeback is a transaction reversal made to dispute a card transaction and secure a refund for the purchase.

Chargeback works by the bank withdrawing funds that were previously deposited into the recipient’s - usually a retailer - bank account and putting them back into your account.

The recipient may dispute a chargeback with the bank if it can prove the chargeback is invalid.

More details here: https://www.which.co.uk/consumer-rights/advice/how-do-i-use-chargeback

Scammers will also often send you a ACH transfer or send you a check to deposit. In these cases many banks will "pre-credit" your account with the funds on the expectation that the money will show up. In these cases it looks to you like you got the funds, your balance reflects the deposit. You then send the item. In a few days the ACH rejects from the source bank or the check NSFs (fails due to insufficient funds in the source account) but you have already sent the purchased item.

This is similar to the scam whereas you are requested to deposit $10K into your account that someone wires to you and you are asked to send another party $2K. In the short term it looks like you got the $10K so you send your $2K but it was never actually there. When your bank tries to collect the money, it fails so the credited amount is removed. Leaving you out $2K.

So, normally any transfer YOU do can't be undone but transfer made to you can fail or can be undone if it can be proven that they were not authorized (a hack or such).

Regarding your last question, you can ask to be paid in cryptocurrencies like Bitcoin. With cryptocurrencies, there is no way to cancel or withdraw a transaction.

That scam usually involves the the scammer using a stolen credit card number to buy the item. If you try to this using your actual credit card (assuming you manage to reverse the payment with the bank), the seller will report the fraud to the police, who will request your personal details from the bank. Depending on the amount of money at stake, the seller may actually decide to go after you for damages in this case, and if you didn't previously reported your credit card as stolen or lost, you may not be able to get away with it.

Seller protection against such fraud often consists in compliance with safety requirements of the trading/payment platform, and insurance.

If the money that was put into your bank account was put there by fraud or as a result of a crime, there is no practical limit to how long it can be reversed. Maybe after six years or so it cannot be reversed anymore.

When someone hacks into my bank account, or steals my cheques, and uses that to put money into your account, that money is not yours, never was yours, and never will be yours. You can even be done for theft if you take it and spend it (and that has happened to people in the UK). It is a common scamming method that some criminal makes money appear in your bank account and asks you to transfer money to them. The bank will eventually take their money away from you, but what you paid will be gone.

As a rule: If someone puts money into your account and then asks you to send money back, it's a scam.