Yup, this is my General order #1. If it feels hinky at all, I pull the rip-cord. Ads that are always a red flag for me, and things like a newish Glock that comes with just one magazine, and nothing else, followed by vague, evasive responses. Generally speaking, no one is going to lose, much less throw away, the relatively nice factory case, and the other factory magazine(s). So when these aren't present, especially in a relatively new pistol, it feels to me like something that may have been stolen from a nightstand drawer or car console, and I bail quick.If you think the deal's dicey, don't buy it.
Like most folks here, I prefer good ol' cash-n-carry, but I won't object to doing BOS's (since that's also kind of a red flag for the other guy), so I've done a few BOS's if the other guy wanted it, but never insisted on one myself. If I got a vibe that made me feel I needed to ask the guy to sign a BOS, then that's enough that I would've backed out already.