I’ll expand on what others have said a little bit more.
The seller has free rein to charge anything he wants for shipping and handling.
As a starting point, eBay provides the seller with tools to estimate what the actual charges might be from the various delivery companies as well as the differences for parcel post, priority, overnight, etc. Estimation criteria include distance, weight, dimensions, insurance, etc.
The seller can then tack on any amount he wants to whatever the result of that estimate is. Or the seller can elect to simply charge a flat rate, again, of any amount.
EXAMPLE: If your purchase on eBay is a slip of paper that fits in a normal envelope and can be mailed with a 43¢ stamp, the seller might, in addition to the purchase price, then charge you a) nothing, or b) 43¢, or c) $10.00 or d) any other amount for the s&h!
The point is, you simply need to include the shipping costs into the total price of your eBay purchase, and decide if that’s actually a good value or not.