I don't play with people who cannot seperate reality and fiction/fantasy - or those who think no one else can. I could not really DM if my players believed that I wanted to become a lich, vampire, or sell my soul for greater power over shadow magic.
As far as slave pricing goes, I tend to run it by the ear. If work horses go for X, then unskilled laborers usually go for the same amount of money. Skilled laborers go for more, but supply and demand and the general randomness of auctions mean that usually there's no fixed price.
Roleplaying-wise, we've had enslaved PCs, PCs freeing slaves, and PCs keeping slaves, all in the same campaign. But then, I consider serfdom as slavery as well, and feudalism without serfs as not that much better, being a rather radical democrat/republican (We Swiss tend to be, with direct democracy, and the ability to vote on anything, from new VAT rates to the purchase of fighters for the air force). So, if I play in a Fantasy campaign, then I can either
a) make it a world where I would feel happy in, without feudalism, knights, kings, and all that, which would make it, basically, to "western 21st century, just without guns or technology, and with magic added in". Not very appealing.
b) play a character from modern times stuck in such a world, reforming it to world a), or
c) I could play in a fantasy world with all that stuff I hate in real life, but have fun playing around in with a character that was born there, and fits in.
