search engines are moving ever faster towards penalising people who buy or sell links (with PR attached)
also, people don't want others checking up on who's linking to them and then finding out how much they're paying for their links
so, you might have more success if you stick to selling links in places like WT and remove the "buy a link here for $3" ad. it's kind of like sticking a for sale sign outside a restaurant - nobody want's to eat there.
also, make more pages! PR4 links are OK but it's worth paying 100 times the price for PR7 links. making more pages will give you an edge over the high-PR sites that sell links whilst having few pages.
good luck 
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