To put this in the simplest terms, let's suppose:
You spend 200DEM at MarketA.
MarketA has a policy where they consider your purchase completed at 6 confirmations.

A block is found and it contains your transfer.
5 more blocks are found and your transfer now has 6 confirmations.

The chain is "reorganized" and the block that initially contained your transaction is no longer "valid".
The "new" chain doesn't contain your transaction.

You can now spend the same coins at MarketB.

If the transaction for MarketB is placed into the "new" chain (and confirmed by the extension of the "new" chain) before your original transfer to MarketA, MarketA will never get paid.
You now have 200DEM worth of items from MarketA and 200DEM worth of items from MarketB; meanwhile, you only spent 200DEM.