I agree with Keys above - appreciate that no one but you knows the order you received the responses.
I would reply:
(written for my own future reference)
>>>>>
“Sincere thanks for your expedient reply.
- I have received an unexpectedly positive response to my offer, so in the interests of fair play, and to assure full transparency and safety for all parties, I will be placing the name in a 3 day public auction at: xyz.com with a starting price of £x,xxx.
(10% LOWER than the asking price, so show you are not unfairly jacking up the price)
The auction with begin in 7 days time at 12:00 on Mon 20th and conclude at approximately 12:00 on Wednesday 22nd May - 10 mins after the final bid is received.
If you would like to participate in the auction, please register at xyz.com and follow their qualification process - to avoid any fake bids or timewasting.
In the event that the final bidder doesn’t complete the purchase within 7 days of the auction close, the name will be offered to the next highest bidder, at their final bid price.
We propose that the purchase transaction will be made via Escrow.com - the industry’s most trusted exchange platform - or via our UK solicitors - xyz & partners - in the same way that a UK property is exchanged and then completed, to ensure we only receive funds when you are in receipt and control of the domain name.
If you have any questions, or require any further information, or to schedule a video conference call, please reply by return.
Yours sincerely
>>>>>
- this then gives you an opportunity to reach out to any other companies that you think could be interested, and also to secure purchase options to buy any similar names from other domainers, that you can then offer to all of the unsuccessful bidders, knowing they have money to spend, and their desired name is already taken by a competitor...