While you can configure the blocks to be 1 pixel, if your grid is too large then your web server will likely run out of resources and not be able to process the grid.
Pixels should be sold in blocks. Selling a pixel 1 at a time would be very inefficient. If you make a pixel for $1, then it could be that the transaction costs would amount to %30 or more depending on your payment provider. Also, it would not be attractive for advertisers, because you cannot put an image on 1 pixel. A pixel is no bigger than the dot at the end of this sentence.