wrote ...
x – [(y*x) + z] = p.

Let's say I upload an item as one of my free monthly listings. Since I only accept payments via PayPal, the only fixed fee I have to deal with is the $0.30 PayPal fee per transaction. Therefore, I enter 0.3 as “z.”

My final value fee (10%) plus my PayPal fee (2.9%) total 12.9%. I therefore enter 0.129 as “y.”

After adding up the cost of shipping and handling and the cost of buying the item I’m reselling, I determine that I need to receive an average of $40 after fees in order to cover my expenses and make an acceptable profit. I enter 40 as “p.”

So the equation I get is: x – [(0.129*x) + 0.3] = 40. After running that through an algebra calculator, I get $46.27 as “x.” That’s how much I need to charge if I want to receive $40 from each transaction.

Need proof that it works?
Here; you'll need this:
Math Papa Algebra Calculator

eBay final value fee: $46.27 * 0.1 = $4.63.

PayPal percentage fee: $46.27 * 0.029 = $1.34.

PayPal fixed fee: $0.30.

Total fees: $6.27. That leaves me with an even $40 after fees.