If someone buys a car on finance (a loan, hire purchase, or PCP agreement), the finance company legally owns the car — or has a claim over it — until the loan is paid off in full. If they sell the car privately before finishing those payments, without telling you, you could buy a car that isn't legally theirs to sell.
In the worst case, the finance company can repossess the car from you — the new owner — even though you paid for it in good faith and had no idea. Getting your money back from the seller afterwards can be difficult, especially if they've disappeared or spent it.
Asking the seller is a reasonable first step, but it relies on them being honest. A proper vehicle history check searches the actual finance records tied to that registration number, which is the only way to know independently of what the seller tells you.
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