Now lets take it to the next step of the thought process. Why might they want to raise fees?

Because they want more money? It's simple isn't it?
Someone cancelling a transaction that's taken place in person with a cash payment is the same as not going through eBay at all.
So by that logic, anyone not using eBay makes eBay raise their prices.
In the real world, it's because they have got the online auction market corned, a lot of people rely on eBay and a lot of people use it for bargains (I've had many a silly bargain from there) so people will still use it even if they put the fees up.