If it's truly under market value, why offer even less? It could be you're response is "no" through ignoring it. Offer what makes sense and reflects your interest in the property.
You can't make someone change their ways, so if you know this person is careless, you and your agent, if you proceed with this transaction, must leave nothing to chance that could hurt you.
Assume they will miss deadlines, if the offer is accepted Assume they will be difficult to deal with, because that is their reputation. If it's not worth the headaches, move on and find another place.