If you are strong enough - is it a source of infinite magic items from his chest for a pretty low level character?
Yes, I know, I need to torture him to make him retrieve the chest, but it is an option for a really evil guy. And you can do it a lot of times.
Yes, I know, that efreet even can cast a wish for you, but there are some problems with understanding the wish and LE outsiders.
But with mercane it is pure profit.
How would you fix it as a DM?
Well, I wouldn't "fix" it as much as have repercusions. You do that to what amounts to interplanar traders, eventually someone powerful is gonna notice things aren't getting where they need to go. And they'll come looking. Or maybe the Mercane had body guards in it's travels, and they got jipped out of their pay, so they come to take it out on that poor low level wizard.
Yeah, the "fix" is that when the person doesn't came back to the place he's expected on time someone sends out a search party. They eventually find the mercane, and it goes from there. If necessary, repeated attempts to free the mercane will occur.
I mean, sure, you might get whatever happens to be in the mercane's chest. Could be nothing, could be something good, could end up with a cursed item the mercane keeps handy for such eventualities. What, you thought mercanes didn't plan for getting mugged at some point? Please. You must have just fallen off the banana 'jammer.*
When you encounter a Mercane out traveling among the Planes he has something to sell or trade. How many are there that don't currently have something though? Unless you have a name, you are just getting a random Mercane that might not have anything.
And of course at some point it returns, and might well be able to hire out a hit even if it doesn't have the power itself to take out the caster.