did you list them as a fixed price per brick as you should have done , or a auction because buyers sometimes never read the description and just bid thinking the opening bid is for all of them. You can ask them to cancel but you be negged and reported non performance
A lot of buyers don't read descriptions fully and may not see the full description on a mobile phone, but even seeing the full description, I think that your auction was confusing. It's not really clear what you're selling. The title says ''reclaimed bricks approx 300'' not ''single brick''.
An auction was the wrong format to use unless you just wanted to auction all 300 in one lot. If you wanted 25p per brick you should have done a buy it now listing with 25p as the price and the quantity as 300.
If you're not willing to sell, all you can do is apologise to the buyer and see if they are willing to accept a cancellation. It's possible that they could leave you bad feedback though and report you for seller non performance.