What are good warranty coverage options for a pre-owned Mac?
Your best bet is probably to look for a third party insurance company that offers policies for used computers. I ran a Google search for "third party insurance for used computers" and come up with one example insurance company, Safeware.

Whether such a warranty or insurance policy is worth the cost depends on how much you acquired your used Mac for and how much you value "peace of mind", since these policies are likely to be expensive and come with fairly strict limitations in coverage.