Merit is for ranking, and trust is for, well, trust. Trying to combine the two would make both systems worse as well as being a total headache.
I also don't see trust needed to rank up, @this time the people who buy and sells on the forum have build a kind of community and knows of each other who's to deal with and when to take precautionary measures before dealing with.... as o_e_l_e_o says and everybody is making tiny trades and giving good trust comments on friends, then the trust system will become unreliable, activity is the main factor in ranking up... only when your really dedicated as a new member you will reach high activity combined with merit to become legendary so even higher will be very tuff as a new member.