Was thinking only 1 or 2 depending on exchange. (I'd recommend Coinbase's value @ time of transaction)
     I understand the wearyness of other forms of payments, if it was me I'd limit it to 2 peices untill repeat business and make them pay as a paypal gift (don't know if they still offer this), that way there's no possibility of a charge back and no fees, then shipping to be paid as a seperate payment through paypal so you can get their confirmed address (non-gift payment).  Most you'd be out is $2 if someone did a chargeback (shipping cost for first class from my experience with AM sticks). Downside is you'd get 2 payments from each paypal address and have to keep track of it.  
     To scale it, I'd only let each person get 2 @ first, wait a week to see if any b.s. happens; then returning customers get bumped up to 4, again a week's wait, so on so forth.  But never anymore than I could ship First Class (AM experience was 6 in a padded envelope (5x7 I believe) @ bellow $3).  That way the only thing you loose is shipping if even that, use the shippig manager in paypal and it tracks everything for you making a chargeback almost impossible.
     The downside to this is that it does not provide any protection for buyers from the seller, and I beleive it's against their t.o.s. too but that never stopped us from doing it that way to non established traders with low/no feedback on the forums.

On another note, why hasn't anyone mentioned the use of "heatware" as a reputation tracker?  Back when I did a lot of forum trading/selling this was and still is the best tool.  The forum one is o.k. but only a 10th of users use it.  Example here's mine: heatware = Tong
It can be used accross multinle boards, provides people a layer of protection as long as others are honest.  I've seen people catch a troll before he gets too far just by using this.  It's a way to rate the experience, as a whole, of the transaction with the person NOT THE PRODUCT.

Tong