After the first purchase from a player's house lot, subsequent purchases do not cost the 20% increase.
So if I need to buy something expensive I'll look to see if the seller has a really cheap item. I'll buy that with the 20% markup and then buy the expensive item for the original asking price that has no additional cost.
So if I need to buy something expensive I'll look to see if the seller has a really cheap item. I'll buy that with the 20% markup and then buy the expensive item for the original asking price that has no additional cost.