Thanks for giving us a nudge on this one Dave, there are a lot of issues in this thread and therefore we will deal with some headlines and then put together a fuller response in one of our next legal updates.
First off, if this is a regular practice of yours Max and you allow sales to consumers (which this person will no doubt now claim to be), then Trading Standards may well become interested and try and argue that you are attempting to get around consumer rights laws. To counteract this you need, as Umesh has highlighted, to hold some sort of evidence that each individual buyer is a trader as Section 2 of the Consumer Rights Act 2015 puts the onus on you to prove that your buyer is not a consumer. I don’t know what is on your trade sale invoices but ours have in large bold font – Important – Notice For Trade Sale which underneath says “It is expressly agreed that the purchaser of the vehicle sold on this invoice is a business and is not dealing as a consumer for the purposes of the Consumer Rights Act 2015”. Further down it then states that “Implied terms as to satisfactory quality/fitness for purpose do not apply to this transaction”. And then above the buyer signature it contains the words “I confirm that I have purchased this vehicle as a Trader/Trade business”. If this sort of wording is in your advert, all the better. We had a similar case in court recently and we persuaded the judge that the advert was transparent leading him to remark that “nothing in the advert amounted to misrepresentation” – in other words the dealer had made the terms of the sale very clear meaning a claim for a full refund plus costs was reduced to a small repair (this was not on one of our invoices exempting satisfactory quality and fitness for purpose hence the repair award under Sale of Goods Act 1979 - which still applies to B2B sales unless specifically exempted).
In short, if your advert is clear and your sales paperwork is clear then you are putting yourself in the best position to defend a claim against these buyers who want a trade price but not the trade risk.
As an aside, you haven’t said if your ad was on ebay classifieds or ebay auction. If auction, then we believe there is room to argue that normal auction rules apply as ‘guv’ Jack Regan has suggested.