Trust is unlimited, I can give out 100 positive trust ratings today, and not lose anything (maybe my credibility, people would stop caring about my ratings, if I just spam them, but you get the idea).

sMerit is limited. I have a responsibility with it. I can use my sMerit to reward good posts, incentive good behaviour with that (publicly shown) reward and help make this forum a better place by increasing the post quality.
Or I can sell it (likely be banned for doing so), incentive spam and contribute to the already very evident problematic of shitposting.

I cant do both, as I have limited ressources. I have to decide, and if I care about this place, this is an easy decision.