S - Seller
Seller - If the seller is a dealer, then buyers have rights of redress under the Sale of Goods Act. However, a private seller is not governed by this act, giving buyers fewer rights.
If an advert claims to be a private sale, ask about 'the car' when you call. If they ask which car, they could be a dealer posing as a private seller. Dodgy dealers do this to gain a buyer's trust or to exploit the fact that the buyer has fewer legal rights in a private sale.