I could have sworn I told PayPal to not DCC me a long time ago but apparently it forgot all of those settings (and I had to Google to find the non-obvious place in the settings to disable it, too). If I had not looked at the eBay checkout page closely enough it would have been a 4% markup on top of xe.com's GBP-USD rates.

On the US site, it's still available if you go to Profile > Payments > My Preapproved Payments > Set Available Funding Sources > Conversion Options. eBay seems to pick this up too as there's no specific setting there that I was able to find.