Potentially they can use the details to set up a direct debit. Some of the less secure sites don’t check details like your address and they could have a DD set up in your name to purchase things for themselves. You are not liable as you didn’t sign/see the DD agreement, but still it’s potentially a faff.
Depends on how much you trust the other person really.
If you’re worried though (and I’d be more concerned where Gumtree is involved), you or they can use a bank mobile payment app, like Pingit, Paym. Alternatively PayPal on mobile or web site.
Either the buyer registers for one of these and sends money to your mobile number or email, or you can request the money and they get a link to make a payment.
(note Pingit is Barclays, but is compatible with Paym that all the other banks use).