We play absolute shares so that there are so many singles and so many "blocks" of 5.
However we also play "friendly" shares. If someone has 5 individual shares and there is a 5 stock available they trade in and upgrade to the five. This obviously won't work if you are playing hidden shares, which I don't.
We play like this also, but in the OP's scenario, I'd just say that the seller was screwed, he could sell all or nothing since noone could actually make change for him.