A lot of the marketplace is supply and demand. For example, the really popular Fairyland event and limited head sculpts will sell for a lot on the MP while less popular and non-limited sculpts will go for much less. Even with yellowing or damage limited dolls can sell for more than their original price if they're still in high demand. If a faceup is done by an expensive and popular faceup artist, it can also raise the price. If someone paid $150 for a faceup they'll probably ask for more than if the head had a $30 faceup. If the faceup was damaged or you were going to remove it, those are factors that may make you hold off on paying more for the faceup. Sometimes there are non-limited dolls with no extras going for more than their original price too. I don't buy these because I don't want to pay more for something I could easily get for brand new, especially if the doll is older and yellowed. If the company has long wait times though, like Lati, then sometimes people are willing to pay extra to get it so quickly. It really comes down to what people are willing to pay.