Starting to get my business inventory up and running

Okay I have posted here before about being unemployed and what is the next step. Thanks for the answers now I have another question?

My business consist of computer repair and cell phone repair. There isn't any cost for me to do computer repair unless its a house call for gasoline or a part that is needed for the customer*. I have been doing cell phone repairs for 5+ years and I am known in my community as the cell phone repair guy. I want to improve and want to get organized to make money I can track. I have always told customers that they will need to wait 3 to 5 business days to get the part in. How can I improve the time for the part to come in. I live in an apartment and I can store stuff in my office space for inventory. How do I go about selecting a vendor to supply the cell phone screens. I usually use eBay or amazon and I just need help keeping an inventory of cell phone screens. Is there any software, tips and tricks and etc. Please help.

I have a credit card with a local bank that the limit is at 2,000 I was thinking that I should at least keep one cell phone part per model ie. iphone5c and Samsung s5. With the money that I receive for the repairs I can pay the credit card off and invest in other business needs.