sMerit per user is finite, unless you're a source, so selling sMerits now will just stop sooner or later because they'll ran out of sMerit quickly. Thus the sMerit sales is not a big concern at the moment.
sMerits are not as finite as you think.
As long as Merits keep moving around (being given out by sources), some users will accumulate Merits and hence sMerits as well.
Sure, no single user can expect to consistently be a sMerit seller by this standard, but wouldn't a good deal of users their Merits if offered something?
If sMerit sales continuously lead to negative trust, some things might happen that lead to more secretive trading.
Like how sold accounts often led to negative trust, hence the account name was revealed only to the other party....
I think this is a legitimate concern that we actually need to think about.