Buying a Used car - Payment

[MercuryT]

Ok guys I'm interested in a few cars to add to mine.

In 20 years of buying (about 6 cars) I've NEVER paid in cash - Bank Cheque always preferred and fine, no worries.

This seller on Merc insists cash only. No bank cheque acceptable whatsoever.

I questioned it as seems strange, but he got upset and told me cash is how you pay.

Informed people here - is this a scam? What's the reason for insisting cash only? I'm not keen carrying 15K in notes in a car with no security.

Thanks all.