When you say “Users” do you mean registered users or visitor to yoour site?
For the first one I use wordpress-multiple-user-ad-management, with some customized code since there a few options that I don’t use. Also the text links options that the plugin have only allow users to create a title and the link for the ads (no other text or description for the ads).
The plugin offers the option for text ads, image ads(but images are hosted externally, user place the source link for the image) and a Paypal option for users to place a Donate button