Yeah, Argentine leather doesn't have a very good reputation, in general. If the saddle fits your horse well, that's a good selling point. I would look at the saddle with an eye to its quality and level of wear. Specifically:
• Is the leather thick and flexible? When you bend it, does it bend smoothly or do wrinkles appear in the fold (wrinkles=bad). Are there cracks in the leather, anywhere on the saddle (also bad)? Poor quality leather stains more easily than good quality leather. However, if the leather is good, minor stains and scratches are purely cosmetic.
• Is the stitching in good shape? If its rotting, frayed or coming loose, its not the end of the world, but realize you will have to replace it (extra money).
• Is the flocking/stuffing in the saddle in good shape? Flocking should be firm and springy with no loose leather, but not so overstuffed that it is rock-hard. Reflocking a saddle can be a major expense, so keep it in mind when considering a purchase.
• Make sure the billets are also in good shape. Its not unusual to replace billets several times in a saddle's lifetime, but if they are worn out, its another additional expense you will have after purchase.
Anyway...sorry, didn't mean to write a book:P Buying a used saddle is often like buying a used car: you have to gauge not only purchase cost, but maintenance costs in the future and judge whether or not its worth the money.
$400 is not a terrible price for a used saddle, but I would expect it to be in decent repair and of average to good quality.