When you buy links are they inboard to you site permanently ? If so then could you buy more from the same company later and get differnt links permanently?
Most link sellers offer sale on Monthly basis, though some of them might sell you a permanent link.. linkworth.com textlinkbrokers.com and text-link-ads.com are sites where you can get what you want and don't forget the dp thread too Abhilash.
Some sites such as www.publishyourkeyword.com offer annual subscriptions. They are inexpensive and you can add more at anytime.
Most sites doesn't offer permanet links. They sell on monthly basis. I think it is better to buy through forum than through link brokers. Because we have to pay their commission. Ofcourse their commission is included in the price. Apart from this i always wonder what a "permanent link" is? How about if the domain expires and the owner doesn't tend to renew?