When looking for beats/instrumentals to use on your project there are specific things you want to be looking for. Although most of the process of looking for beats is based on personal taste there are still a few guidelines you should follow:
Good Quality
One of the first thing you should be looking for in the beats you are looking to purchase, is if it is a good/decent quality. It's not necessary for it to be completely mastered because the beat will be mastered with the song that you decide to record. What IS necessary though is that there is quality arrangement and quality mixing. If you're having trouble noticing what sounds good and what doesn't then try this technique:
1) Find a song that you like that has a similar beat to the one your thinking' about purchasing.
2) Play the song you found 1st then play the beat 2nd.
3) While you play each notice the mixing and the arrangement and compare the 2.
Good Sounds
Once you check for good quality, the next thing you want to look for when coppin' beats is good sounds. You should be looking' for:
1) Catchy melodies. (Easy way to find out if it's catchy is if you can easily hum along 2 it.)
2) Bangin drums,
3) Sounds that mix well. More often than not producers (especially beginner producers) layer sounds on top of each other that don't really blend well with each other both pitch wise and frequency wise. This is a little easier to spot because the beat will be very hard to listen 2.
Good Prices
As the artist you should be looking for quality prices to go with those quality beats. Some good price ranges if your shopping for leases is - if it's any more than that then they should be bringing something extra to the table that's worth more. So for example if their beats are out of this world and they got placements with Young Jeezy or Lil Wayne than it might be worth the extra bread, but if their beats are meh and they don't have any placements, do not waste your money. Some good prices for exclusives on the net are 0-00. Considering a lot of big name producers charge at least 10X that I would say that's a pretty decent price range.
So to wrap this up make sure when you buy beats that they have Good Quality, Good Sounds, Good Prices and you can never go wrong.
Also if your looking for even more detailed info on how to buy rap beats then check out http://www.howtobuybeats.com