50% for sale @33.3% markup
must sell 100% of that 50% (7,5chips) to play.
player pays 7.5 chips to create stake
receives 2.5chips returned as markup (33% of the 7.5 chips of stakes sold)
Net result player pays 1/3 of buy in for 1/2 of win
This is a fair offer to both player and buyer, and allows player to stretch his bankroll buying in a a bargain price.