Buying or selling links is against the webmaster guidelines.... Usually links offered in such deals are spammy and wont give you any benefit. SE are smart enough and can easily catch you for this. Its better to opt legitimate linkbuilding to avoid any kind of penalty or ban.
You will only get short term gains from link buying and you will need deep pockets to maintain any links that you buy. As C.Rebecca pointed out, it is much better to use more legitimate methods of link building.
This short term link buying is how much short 3 to 6 months or to short. Is these links known as advertisement links or similar like backlinks.
Agreed. Look at what happen to J.C. Pennys - they were buying links - Google lowered the boom on them.
I think link buying is good if you use quality linking services but you can also build backlinks by yourself. That might be the best solution if your website is not too big.