Unless you have the neccesary coding skills in house you will need to pay someone to do it for you, however, cos i'm a nice guy here are some ideas to get you started:
- A database is pretty much essential for this, i would recommend 3 tables:
user
- usual user details
- number of credits/money
transactions
- user
- download
- comment
downloads
- title
- description
- cost
- Next you will need some PHP logic to do the relevant listings and reduction of balances when a user downloads a product.
Safe :-)
Tom