Personally, after 20+ years on Ebay, if I am asked to cancel I simply do so. There is no point in getting cross or stressed or getting into a back and forth with the buyer. Life is too short!
However, what you can do, if you choose, depends on whether or not they have paid.
If they haven't paid, you can ignore the cancellation request, wait 4 days and cancel on day 5 using the reason 'buyer did not pay'.
This will given them a non-payment strike on their account.
If they have paid, you can refuse to cancel and send the item. However, if you do this, the buyer may well find a reason to open a case and return the item.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)