They may have been referring to one of two things.
1. They may have meant that the Buy/Sell/Trade forum will sometimes have members selling or trading for prototype pieces. These are member-to-member transactions that are done through the forums rather than down by BZPower itself.
2. Around 2009-2010, BZPower was given a decently large quantity of prototype parts by LEGO. These were test molds that weren't originally intended for public release, and many of them were co-injected two-color versions of previously single-color pieces. They were initially given away as various contest prizes, to members who attended LEGO conventions where BZPower had a presence, and to members who mailed their MOCs in to be included in convention displays. Unfortunately I don't think there are any of those left to be given away, since it's been many years and many members have earned them. One of the conditions of BZPower giving them out as prizes, though, was that those who received them were not to resell them, but I don't know if that rule has been relaxed since they've run out. Pretty sure you're allowed to trade with or for them, though, but that goes back to #1 above. Here's photos of them; I think there were multiples of some.