Starting a Small Cellphone Business

I am trying to start a small scale cell phone sales and rental service.
The service would consist of:
-sales of tracphones (go phones, pay as you go, burner phones)
-rental of "smart" phones
- catering for about 100 people
- small scale repairs
- sim cards and minutes would be provided with most phones

- sales of tracphones
I currently have a range of tracphones from $8- $20
(price varies upon condition from very used to brand new)

-rental of smart phones
I currently have a range of "smart" phones (blackerries- droids- iphones)
(price varies upon condition from very used to refurbished)

- catering to about 100
This is self explanatory but they are all University students

- small scale repairs
I am currently accepting donations and whatever phones I can find for cheap. Most of these phones come in a condition from battered to small problems such as a new battery

- Sim cards and minutes would be provided with most phones
Almost all phones I hand out would have a sim card with $10 credit (which is roughly 50 or so txts and about 10 minutes talk time)

Ok so, my questions are:
does any one have any recommendations for me?
I am open to critiques or suggestions as to how I can make this work better
Lastly, I am in a Caribbean country which is GSM only so I assume that Sprint and Verizon phones won't work here...

thank you for reading
(I am new to this whole forum so bear with me)