Short answer: Not much.
All the above have a Blu-ray release and none of them are rare or OOP. You'd be looking at a few pounds each, maybe £10 or so for the series collections if you're lucky.
CEX, MusicMagpie etc are likely the only way you'll be able to sell them.
I'm curious too. If UP1 are matching Right Stuf prices but without the significant shipping costs or import fees then that's fantastic.
If they're merely acting as a "proxy" then it could work out more expensive than buying direct
More information is required.
Fedex, UPS and possibly other couriers don't bill you in advance of delivering the package, they send an invoice afterwards. I know from experience that not paying gets the bill referred to third party collection agencies.