The only problem with giving out something like a serial number is that it helps someone who wants to scam people by making their adds look better. People sometimes post their serial number, or a picture of it, in an ad and that is the perfect opportunity for someone looking to set up their own fake ad to make it seem more authentic. When you see these fake ads, they are often copied from and old ad somewhere because when they aren't, they are pretty obvious scams. Unfortunately, you have to be paranoid to some extent, but still buy the majority of my own equipment used. While the vast majority of people out there selling equipment are good people, there are some bad ones. You just have to do as much homework as possible in order to make you comfortable with the seller. Asking questions here can be very helpful.
The biggest red flags in my book are:
1. Price. If it is too good to be true, it most is.
2. Odd payment arrangements. The safest method that a seller (and buyer) can use are PayPal (or similar) or a credit card. When the only thing the seller will take is a cashier's check, you should probably back off since that is essentially the same as sending them cash and there is no recourse if it is a scam. Wire transfers can be problematic that way as well. But I have sold things to people who paid with a cashier's check and things were fine. I just waited to ship anything until I was sure that it was good.
3. Odd delivery arrangements. This guy said the mount was in Europe but he lists himself as being US-based. At the same time, appears to be going back to the US soon. The location situation might raise a red flag, but if you combine it with any sense of being in a hurry to make the sale, that is definitely a red flag without any further explanation that you are completely comfortable with (e.g., maybe he was just there to install his new system and doesn't want to haul the old one back with him).
4. Odd item descriptions that might appear to be from someone who doesn't write English well while other things, like maybe their address and phone number, suggests that maybe they should. This alone is not a good indication as I work with people all the time whose English writing isn't great. But if you combine that with any of the other possible red flags or something that suggests that the description might be completely drawn from a manufacturer's website rather than any indication of personal knowledge of the item, then it becomes more important.
None of these things are show stoppers and there may be good explanations for all of them, but they should probably raise red flags that you should want to look further into before agreeing to the purchase. No one should be insulted by your asking things to verify the authenticity of the sale in this day and age. If they are, walk away.