That's why there is more or less consensus over it: if some buy or sell merits, it means they trade a measure of recognition on the forum and it makes them dubious counterparties. So trading with them has higher risks, so they deserve red marks in a trust system.
So right, it is not a matter of rules, but it is a matter of reputation system of the forum.
In order words, if the word 'trade' is not involved. I mean there is no monetary attachment, it is right to swing into the dm of a merit source and give them reasons to be dropping me merits more than I deserve. If this isn't seen as a merit begging by the merit source, it is all fine that way.
This simply means there are no firm rules in BTT.
There's this:
I don't believe in having a set of hard rules which is to be applied to all cases. Whenever an argument starts looking like it was written by a lawyer, or relying overmuch on precedent, you've stopped thinking about the real case and have started using rules to retreat into moral and intellectual laziness, divorcing yourself from the decision you're about to make. If you're making a decision about a case, then you're responsible for that case, and you can't say, "I don't agree with it, but I was just enforcing the rules." Every case needs to be handled individually.
I can confirm today that theymos is a wise man. If there be rigid rules for everything, the moderators would likely be acting against their wish in most cases. Because they wouldn't use their reasoning or conscience, rather they will simply trace your offence to a laid down punishment and that settles it.