I sold a PS4 to CEX and they said they didn't have enough money in the till!?

CEX branches don't usually hold much cash in the tills for security reasons.

It also persuades more sellers to accept store credit instead, and that's more profitable for them as it guarantees they keep the money when you spend the credit with them. It's sufficiently profitable that they always pay significantly more when buying goods if the seller accepts store credit.

But if you want an outright sale then they'll always offer direct and near-immediate payment in full into your bank account. So if you refuse that and still want cash then you have two choices: allow them time to get the cash to pay you, or take your goods away to sell somewhere else, maybe even a different CEX branch. Not Cash Converters though as they always pay ridiculously low amounts.

I've used CEX for about five years now,
Mostly the Croydon, Wimbledon and Streatham branches (all in south London), and they've always been trustworthy in my experience.

If you're not convimced just return and get your goods back

It happens sometimes. I assume you made sure to get a receipt and a credit note before leaving the shop?