How to classify eBay sales and PayPal income on GNUcash properly?

I have to say that I am just beginning to learn gnucash but I am also just beginning to understand double-entry accounting. (This was occasioned by my having been asked asked to be the treasurer of a non-profit organization I belong to.) So please make sure you double-check my answer below with someone more knowledgable.

You cannot consider €9.75 to be Income and €0.25 to be Expenses:Fees, because for reporting and equity purposes, Net Income will be computed as Income – Fees, which would come out to €9.50, whereas it should come out to €9.75. Since Net Income will be used to compute taxes at some time in the future, you will be under-reporting Net Income and therefore underpaying your taxes. both of which are bad for business, albeit in different ways.

So I'd do a split transaction something like this:

Income:Sales increases by €10.00 (Credit)
Expenses:Fees increases by €0.25 (Debit)
Assets:PayPal:Cash increases by $9.75 (Debit)

Of course, I don't know exactly how you break down your accounts, so you might have different names for some of the subaccounts.

By the way, I've found the official Gnucash.org "gnucash-users" mailing list to be extremely helpful. This link gives instructions for joining. You can then get much more expert responses on both Gnucash issues and accounting issues by posting your query there.

I hope the above is helpful.