AbdulAlhazred
Legend
IDK, the non-metallic alternatives in Dark Sun seem adequate for survival purposes. Magic items get very expensive very fast for being only marginally better than ordinary weapons. You could certainly trade a +1 sword in the survival economy, but it probably wouldn't buy you many more survival-days than a regular sword (and it'd probably make you look pretty desperate).
In the 'separate economy' model, you could place gold (or platinum or astral diamonds or Spican flame gems or unicorn rainbows or whatever), it just wouldn't have the theoretical astronomical buying power in the more bartery survival economy, most of the time.
Of course, there could be a cross-over between the two economies in major centers of civilization where the very-rich-in-both-economies do business.
Its always possible to use an item from the 'magic economy' to cross over into the 'survival economy'. Will the water seller give you a large flask of water in trade for a Wand of Fire? Heck yeah! You better be pretty desperate to do this, and the deal may be surprisingly difficult as he may need proof that it is what you say it is. In fact, you're probably better off to go to a fence, etc and trade your magic for some coin, but you'll get FAR less than you'd get dealing in the 'magic economy'. I'd say, in general, you CAN unload anything for coin, but you probably cannot BUY just anything for coin. Most significant deals are going to be either covert, or regulated, and probably involve trades. You might trade a building for a magic item, but you won't get it for gold.