Forget the fact that its against Facebooks TOS, you shouldn't do it just because is actually detrimental. That is, if you buy likes your paying to have a lesser chance of doing well. Let me explain.
1) First and foremost when you buy likes your getting likes from fake accounts, bored kids, people who have no interest in your product, and bots. The whole reason of a fan page is to build a community that will have some interest in your product or service. With that in mind what good is having likes from people who will never have any interest in purchasing your product or service? There is no good in it, its just a total waste of money.
2) Even worse than wasting your money, you have to consider the promoted posts. Promoted posts are posts where you pay Facebook and they make sure more people will see your post. This is great if your having a special, or make a post that you just need people to see. Promoted posts are sent to your followers (people who have liked you) and to the friends of your followers. The cost of a promoted post is also based on the number of followers you have. More followers (likes) more cost. So if you buy likes than your cost to promote a post is higher. So your paying more to promote to people who are not interested in your product.
Example: if you purchase 10,000 likes a $0.03 each then you paid $300 for those likes, a waste from the start, now lets say you somehow acquire 250 real followers, people who are interested in your products. You are having a special and want to promote a post to your followers, with 10,250 total likes / followers your cost to promote a post will be around $40. But 10K of the people your promoting to are a waste, if you had just the 250 real people to promote to, the people who are actually interested in your product, your promotion cost would only be $5
3) Buying likes will make you look bad! What you need to understand is that like sellers only care about hitting the target number, once they do that they have earned your payment. But they make no guarantee that those people are not going to unlike you shortly after and believe me they will. Your going to have to trust me on that, but please believe when I say fanpage fans who you paid to follow you are not going to follow you for long. So what do you think happens when you start getting unliked in droves? When real people come across your fan page and see that 15, 20, 30 people a day are unliking you it makes it look like your a bad page. It hurts your reputation.
If you want to waste your money there are much better ways to do it. Grab some beer, go play poker, send it to me, whatever. Just dont buy likes, your not only wasting money, your countering everything you hope to get from Facebook.