That's a good point, but I also look at it this way. If someone has enough btc to pay me upfront for an item and share me their shipping details, and has a decent rep in collectibles or is anewbie who is actually trying to learn/get ahead, (without any shenanigans or feelings of unease), and I deliver and they are happy, I see that as fairly trustworthy, as ofc I have no reason to get scammed, nor would I scam anyone as most people who really know me know. To be fair/honest, sometimes I never leave trust in this scenario, and sometimes I do if it's a high-value item or something "different", so I don't think I've ever enabled any scammery in this manner .... I will think more about this as well.
As for the second scenario, yes, it indeed does prove trustworthiness, and I usually put trust on those deals ahead of the former. However, I have not been part of many deals where half is first/half later for some strange reason.
Edit - Either way, I think a lot / very carefully before adding positive trust. In fact I sold Rainbowsky a coin thinking he was a newbie just trying to learn. He had me send a bunch of pics of coins I now know he could never/would afford, then just bought only one cheapo for like 80 bucks. There was no way I would leave trust for a trade of that value like that, and I actually figured out it was Alamjob since he had sent me something once in a raffle or something small, and I just happened to recognize his name/location as being the same as alamjobs. ... That's how I figured this all out. In fact, I gave Rainbowsky a merit once, thinking he was just a newbie trying to work his way up, since I thought his post was actually good/true.... Little did I know he was just "behaving" and acting "normal" at that time before being "outed" again ....