Depending on the request style.
1. Alleged wrongdoing? (in one case, they even Cc'd: to registrar abuse@ email) --> No response
2. Stories how poor they are, detailed explanation how they paid but their provider did not renew it, "please give the domain name back" --> No response in most cases.
3. Anything else - common response [not changing the asking price] with an extra part ("domains are not owned but leased, if you elect not to renew the lease then...") as well as confirming current expiration date to make sure they'll renew next time after this potential purchase.
In (1) and (2), "no response" means that I internally mark the leads as "not serious". I simply have no time to respond to such leads.