It is important that you are prepared if you are going to ask your boss for a raise. Make a list of postive things that you have done for the company. Relate that to fellow workers who do the same job.
So if you have been there for 8 months and have the same areas of knowledge, are trained in the same thing and also can prove that you are out selling those people making more, those are your strong points.
Give some negatives as well. Give him weak points that you recognize and are working on. Show him/her how you are working on them and what tasks and training you are planning or have done to improve those. You can't just be great...you have to show that you have limitations and you see those and are working on them in a timely manner. Give a long range goal for yourself too. If you want to learn another area of the store, give a timeline of when you plan on getting that done and do it.
If this sounds like too much work, then you don't deserve a raise. Companies do give extra money, if you show that you are working for a long term job at a place, they will help you and sometimes increase your pay.
Do I think your going to stay there longterm? Of course not, neither do they, but they will still support you in hopes that you do.