I think some big assumptions are being made here. Yes, there is a TON of low quality junk at flea markets. But mixed in with the SAK knock-offs, you will frequently find a Victorinox or Wenger. Often at the same price, since the dealer might not know or take the time to sort knives bought in lots.
Likewise, I've seen Solingen and Pakistani knives on the same table. Frequently, these are used knives, not new. But new ones exist too. I passed on an AG Russel bowie that the dealer wanted $60 for because I didn't think it a great bargain.
The bottom line is, if you have done your research and know what is good and what isn't, brand and quality-wise, and you inspect carefully before buying, you can hit some real bargains.
Cliff's comments apply, too, in that if you're buying a no-name, you may regret it.
My $.02
Mike