In my own marketplace I'm searching for a way to reward outstanding customers, f.e.

• a customer purchases paid downloads worth more than f.e. 100 USD during a year (financial reward)
  -> reward the customer with 5% account credit of the money spent
• a customer purchases paid downloads from 10 different developers/authors (community supplier reward)
  -> reward the customer with a special paid product for free
• a customer purchases all paid downloads offered by a "featured" developer/author (downloads bundle package reward)
  -> reward the customer with the cheapest product (money back as account credit)

I'm selling downloads, that are either exclusive or can be purchased from other sites as well. Hence I'm attempting to reward my members buying from my site instead of competitor sites with some credit rewards as a "thank you" mechanism or other benefits. Important: Without discounting the products! It's a gift by me to primarily choose my place.

This could be a nice feature for the IP.downloads application.

Has anyone realized such rewards in paid downloads so far?

It shouldn't be hard to do on concept.  Move the buyer to a secondary usergroup when he purchases the file (or meets your criteria) which gives them discounts or rewards on their next purchase.  

Everything you want can be done using Automation Rules by @Kevin Carwile