Okay... this is how I read it... if I'm wrong admin can come and correct me and tell me I'm an idiot, I don't mind.
Say you have a hide-a-bee and you offer it with a value of $7.... I would request that from you, you'd send it to me and pay for shipping, get tracking and send me the tracking number within one week.
You would then have a $7 credit... say 2wks later Twalla posts a MD tshirt with a value of $7 and you wanted that you could use your credit you acquired from sending me something to request that from her. Twalla would pay for shipping and send you tracking information and then she would get a $7 credit.
I imagine members could build credits up... say you sent out 4 items and the total for all those would make up $20, you could request one item that was valued at $20 or you could use the credits in say a $12, $3 and $5 trades.
The idea is that it works on a honor system, so we basically have to trust each other and not fiddle the credits. Other members would probably keep track of peoples just to help out.
Now I've just wrote this I can see how people would try to take advantage of this... I'm not pointing the finger at anyone in particular but we all know we've had members turn into scammers on YT in the past.