I believe you have over-reacted, and in high fashion. This is much ado about very much nothing.
(1) In some countries, a law firm can press a copyright
claim even when they do not represent the copyright holder. So there are law firms that have whole divisions that do nothing but mail out letters like this one and look for a suit to file and a settlement to bank.
(2) Some companies who own copyrighted property that is widely pirated have law firms that just send letters (and ultimately sue) known violators. The maps that go with chart plotters have been widely pirated for many years. (If you buy or are given a copy of software
rather than the original, you are a thief. )
(3) Either way, If you are reselling the original, and not a copy of it, then they're just trolling because they saw your "for sale" ad and targeted you as a software
pirate. The original product may be resold, and I'm pretty sure this is the rule
everywhere. But it's worth noting that Garmin might not provide any customer support to second and subsequent purchasers.
(4) I am not a lawyer, and everything here might be wrong. If you seek real advice on a legal
matter, consult a real attorney on it.