Remember that not every eBayer has the choice to 'buy locally'. EBay is a useful resource for customers who have limited choices in retail outlets.
There is very little reason for urbanites in good health to buy, I dunno, tube socks or USB drives, or computer parts online. But for those in rural areas or those who need specialty items eBay is a godsend.
And of course, collectors find all kinds of neat stuff here that is not available anywhere else.
You can ignore any seller using the GSP for items under $100 (import fees kick in at $20) by watching for
Customs services and international tracking provided
on the listing. GSP items will always have that note.