I agree with boxerdog on this one, but appreciate the confusion.
From my (basic) understanding of contract law, inviting offers does not result in contract unless the seller accepts the offer. If this were an auction then the highest bidder would be entitled to the domain, but this thread is offers invited and there is no indication that this is an auction.
Even if the domain was put up at a fixed price, for example 'domain.co.uk - £50' then this is an invitation to treat. If a buyer offered £50 then the seller would be under no obligation to sell. That said if the post stated 'domain.co.uk £50 - post sold to claim', then the contract would be binding if the buyer posted 'sold'.
I can understand the confusion here, but placing a minimum offer amount stops lots of low offers coming in. However, maybe for the future use guides such as Low £xxx, or Mid-Low £xxx.