I would like to think that section 75 of the Consumer Credit Act offers a protection. One should be able to claim against the credit card company. The card companies would pursue the seller; if the money is beyond recovery, the insurance payout should kick-one.
The offer under discussion might be genuine — I am under the influence Morphine now, or suspicious. If not the former, I do not think the £100 is relevant.
Of course, I have no prior experience of buying like this. But, have had calls from a card provided alerting me to verify transactions - irrespective of the amount.
I think the real risk is not £90+ but your PII and card details.
Paying by debit card is a different story.