Someone much earlier in the thread mentioned geocaching as a way to execute a trade.

I thought of another way.  There's this group of people who do this thing called "geohashing" (google it).  Basically they take a hash of the date plus some figure not known until that day (in their case, they use MD5 and the NYSE closing price) and compute a lat/long with it.  And they meet up (whenever possible) at that place.

Something like this - merely the idea of a pseudorandomly selected meeting place - could make a trading system viable, nobody would need to go to any known specific place... (that is, after the algorithm being refined so it chooses places people can actually go inconspicuously, rather than in the middle of some farmer's field).

Further, it could be arranged so that the pseudorandom location generator had some sort of duress input, whereby a drug user turned informant by force could simply give a duress input that would break the function of the algorithm and leave the cops showing up to a random place all alone...

Just random ideas, I don't see them as viable all by themselves without major refinement