Can I sue for price difference of item sold on ebay

I bought an item on ebay and it was not delivered. I have requested the item but instead the seller has issued a refund as soon as they could.

Assuming the item wasn't actually sent then can I sue the seller for the price difference between that item and a simlar item online?​ for eg. the item was sold at a fixed price of around 100 pounds and similar items may go for c.250 pounds. (although some have gone for c. 125)​

I requestrd the item and not a refund, but I have been refunded the full 100 pounds I spent, can I now sue the seller for the difference above and beyond the refund ?- i.e. 150 pounds ​(250 minus the 100 refund ) or would it be the case that I can only sue for the cheapest pair that has sold online - ie. . 125 minus 100

What can reasonably be taken as the loss difference- any item or the cheapest?

Can the seller say they sold it in error for 100 pounds and this should have been noticed by the buyer - and then cancel the contract?

No contract seems to have been agreed and none can be found on ebay - does it default to a standard contract somewhere?

p.s. in a small claims court who would pay for the fees? Defendant or the one who has sued? Also if I live in London and the seller lives in Birmingham, would the case be tried in their home town or mine?