You can trade BMIs for other BMIs!

For example, you can trade one Nyx Pack for 3 Titan's Challenges.

Essentially you can trade a BMI for the 'pass equivalent' of other BMI, so since a Nyx costs almost 600 passes to buy and Titan's Challenge is 200, you get 3 Titan's Challenge

Thank you! I know in the item exchange you have to have a Pegasus/VIP account to trade more than 2 items at a time- is it the same for the Scale?

No, the Scale lets you trade item in exchange for that of equal value, so the Exchanges limit does not apply.

As in the example above, a Nyx costs about 600 passes in the Black Market, so what you can pick in return is worth up to 600 passes, regardless of the copies it takes to equal that. If you wanted a Golden Apple, you'd get 6 when giving up a Nyx, because 1 GA is 100 passes. 6 of those equals 1 Nyx.

Thanks folks!