Unlike patents, trademarks have to be actively defended to remain valid. My guess is that companies of this size send out letters like these to maintain that they're indeed defending their trademark.
I wouldn't worry about it but still send a reply letting them know why I don't think their trademark applied.
It's a little late to say this, since you (ebaysucks) have posted this in public (bad plan), but DO NOT REPLY under any circumstances. Replying does two things:
1. It alerts the sender that there may be an interested party on the other side of unknown intentions. This gives them further reason to continue to pursue that they didn't have before.
2. It confirms to the sender that the message was received by a potential defendant. That, then, becomes a piece of the litigation: "Your Honor, over a year ago we sent Mr. So-and-So an email regarding this matter, and he flatly refused to negotiate in good faith with us. Given that... (insert bastardly thing here)."
In short, don't reply, and never reveal your thought process to them except through counsel. And don't pick painful fights (especially inadvertently) for no reason at all. If you feel an overwhelming urge to take some kind of action, talk to a lawyer.
I wouldn't worry about it but still send a reply letting them know why I don't think their trademark applied.