It is not clear what you mean by "the seller asked for this".

As you say, the items you "asked for" were £2.40 each, with free postage. 

There seems to be a lot of asking for here! When you buy something on eBay you are entering into a contract, not a negotiation. You are not required to pay more than the price for which the item or items sold, except when there may be import duty or VAT to pay. But this shouldn't apply here and it's hard to see how the price could have escalated to £28.

Have you actually bid for and won these items? I presume you questioned this before paying? What was the seller's response?

Hard to help with so little information.

Without seeing the ad. it is impossible to tell, how the price seems to increase far too much, in any case if it says £2.40 and you order 4 then the price is calculated by ebay not the seller. Give the item number?

The seller is based in China, and reading their feedback says it all about price hiking. You have been warned, shop elsewhere.