Don't know, but to me it feels like hiding costs to make a little extra money. Kind of like charging $15 shipping and handling when the actual cost is more like $5.35 or something. If I want $40 for my part, why shouldn't I ask the buyer to pay the fee's if he wants to use PayPal?
If I ask for it up front, he can decide if he wants what it will actually cost (part+fee+shipping) or not. If not, then don't buy it. In the end, I feel better about it because there are no hidden costs.
If I price it to include everything, then I get accused of being a "car stripping profiteer".