It's been a very long time since I've dealt with this issue so thing may have changed.
But first, to be clear, there is no way on eBay to do what you want, items are either taxable or not, there is no way to have variable rates.
Not sure if it still works this way but books at one time in Ontario worked on a rebate system, 13% HST would be charged by the merchant and then an 8% instant rebate would be applied. I suppose you could still do that by issuing a partial refund post-payment.
Personally I only have one or two book titles for sale, rarely do they sell to Canadian buyers but when they do they get hit with the full whack of HST. Keep in mind that only a few provinces have HST (primarily Ontario) so the amount of sales where this issue comes up is quite limited for most sellers.
If a buyer were to question the amount of Sales Tax charged for a book purchase, I'd be happy to refund them the overage but I've never had anyone complain (a very small sample size).
Maybe an actual book seller will chime in on this............

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
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