You can Pm them whatever you like. Some take great offence at their prices being questioned, others appreciate people pointing out that their price may be wrong. Lowballing may be a different issue as may be seen as taking the pee. You've also got to consider whether the two watches are really comparable. An eBay speedmaster treated in their "in house spa" etc will not be comparable to one with service paperwork from say STS. Condition will also vary between the two, and unlike Ebay, sellers here will generally point out every possible flaw, not hide them with clever photo angles and just say condition is commensurate with age etc.
Basically no point making a comparative offer unless you know exactly what you're talking about.