Suppose you have an index and you think that raising the index probably improves whatever you really want. Then you set up some markets which say decision A probably raises the index and decision B probably lowers the index.
Now you have an explicit map {decision A/B} → {index}, and an implicit assumption that your true objective goes up and down with your index, and from these two things you could conclude that decision A is good and decision B is bad. However, whether decision A is actually good depends on whether the implicit assumption about the {index} → {true objective} map is sound! I think in most situations you can do at least as well by explicitly estimating the {index} → {true objective} map instead of leaving it implicit, whether you do the estimation with markets or some other technology.
Ultimately, you want a better estimate of the {decision A/B} → {true objective} relationship. This might be a hard problem, and all solutions might not be great (our suggestions might help, but the answer might still be unclear). Even so, there’s no way to get around the fact that this is the problem that needs to be solved.