What is the best way to allow one site user to pay to another user, through web site using credit card?
What is best fits for this Paypal Pro or Authorize.net?
You can't do this with Authorize.Net or Paypal Payments Pro because they offer no service for this. If you are a US based merchant, and you have to be to use Authorize.Net, you can't use your merchant account to accept payments on behalf of another businesses. It is called factoring and will result in your merchant account being shut down and your business, and you, being added to the Match File which is a blacklist that prevents you from ever having your own merchant account again. T
To accomplish this you will either need to use something like Paypal's split payments or force your users to all have Authorize.Net accounts. If they all have Authnet accounts you can dynamically use their accounts to process payments. However this means you will not be able to deduct any fees fro the transaction and will need to run another transaction to charge your users any fees incurred for providing your services.