At the end of the day, in effect, if you're a small-supplier/personal seller, let's say you're in Ontario and sell your dad's old fishing rod to someone in Ontario for $100 and deliver it yourself.
You'll pay ebay ~13% ($13) in fees, and they'll charge you 13% on their fee in HST (another $1.69). Starting July 1st, ebay will charge the buyer 13% HST too, so they'll pay $113 for your $100 item, for which you'll get about $85.30.
In total, that $100 transaction will generate $14.69 in HST that ebay will helpfully pay CRA for everyone involved.