When the purchase is made from a 'Trader' the buyer has far more protection under the law then they get when buying from an individual.
For example, it is a Traders responsibility to ensure that your purchase is correctly insured when they post it.

As with any set of laws there are often exceptions and one is that while Auctions are not covered by the Distance Selling Regulations, items sold for a Fixed Price are covered.

It can sometimes be difficult to tell Traders from Individuals, eBay introduced identification of Sellers who were registered businesses to help buyers make the distinction, in October 2005. There is more about this here Business Identification including details of some of the related laws.

In some cases, where there is no formal indication that the seller is a Trader, it may be that the volume / nature of their sales makes them one in the eyes of the law.
As a rough guide the following might count as Traders

• Those selling large numbers of the same item or type of item
• Power Sellers
• Those offering lots of items for sale at once
• Those with high feedback, especially if most is from other buyers

If you are any doubt about a purchase you are advised to take independent legal advice.
Your local Trading Standards Office may have a helpdesk or you could try the Citizens Advice Bureau

Legal Guidance for Business Sellers