Payments and user balances/virtual coins
Yes, Userpoints should work for this, with one major exception: It doesn't support decimal values. There are patches available for both D6 and D7 but you're pretty much on your own when using them.

There is integration with Ubercart available for D6 and Commerce for D7 (http://drupal.org/project/commerce_userpoints) which allows you to buy points with money. There are also modules available in the Userpoints Contrib project for requiring points to create nodes, which you can combine with Userpoints Nodes and Comments to remove points when a node is created, resulting in the ability to sell creation of nodes. And, there is Userpoints Node access, http://drupal.org/project/Userpoints_nodeaccess, which allows to buy access to nodes (only the D7 version works).

In general, the D6 versions of the modules aren't well maintained, especially the modules in Userpoints Contrib are very unstable (while those ported to D7 should work pretty fine).

Disclaimer: I am the Userpoints D7 maintainer and might be biased.