I'd also be glad to know if there is an official rule on the matter and I missed it.
Only for "merit sources (unfortunately)" but that doesn't mean it's fine for other users to sell/trade/share their merits:
"You are a merit source. The next XXX merit you spend will come from your source rather than your sMerit balance. Merit spent from your source will come back in 30 days. Unused source merit is wasted. It is not allowed for merit sources to sell their merit."
I found this quote slightly disturbing - it would tend to imply that others who are not merit sources are allowed to sell their merits. That rather kills the whole concept.
Good catch, OP, but this belongs in reputation.
Or here:
Suspected users that are abusing merit 3.0