You have already done the right thing you have swallowed your pride.
The problem with selling online your personal items is that only you know the condition of them when they left you - no one else, not ebay, not the buyer. The buyer is the only one who knows the condition when it arrives not the seller ebay or anyone else.
Again if the item fails 'breaks' soon after purchase only the buyer knows how when and where it failed not the seller ebay or anyone else.
Therefore you have no choice but accept the buyer's complaint unless you can solve it for him at distance, in the same way that the buyer accepted your photograph and description on ebay.
With regards to the buyer's abuse, you need to first look at how you handled the initial complaint - the buyer was probably upset angry, frustrated - if you met it with a head on - you broke it, you are lying, I do not accept returns, it was in good condition when it left me etc - you may just have fuelled the fire.
It can seem personal - people typing at home both sellers and buyers become keyboard warriors -
However if you remained polite, showed empathy and solved the problem reasurring the buyer that if they returned the item you would refund them or even possiby refunding them without a return if the sale price was low - then you have every right to feel indignant - in which case you can report them to ebay who will review both sets of messages and take a view on whether to sanction the buyers account or not.
