You can use topic type sponsorship to let people purchase a more eye-catching link, or show them in a special place that regular topics aren't shown.
First, fill out the general settings at the top of Admin Panel -> Revenue -> Sponsorship. You can use any of the three supported payment processors -- PayPal, 2checkout and nochex -- or you can leave all three of those fields blank and process payments manually.
Click the "Add Link Type Sponsorship Level" link to add a level. Follow the instructions on screen. If you want to use a new topic type you'll need to set that up at Admin Panel -> Topics -> Link Types first. After adding the sponsorship level, you'll see the levels you've created listed on the sponsorship settings page for you to edit.
If you want to require all topics to be sponsored, just set up sponsorship levels for all the topic types you have.
You can customize the payment buttons and the rest of the payments page at Admin Panel -> Themes -> Manage Templates -> "Sponsor Purchase - Link Type". You can set the price in the paypal button to whatever value you want, and that amount will be credited to the account on purchase, and deducted at the rate specified by your sponsorship settings. In other words, feel free to have people buy in $100 quantities even if you want to deduct at $1/day.
Prompts asking people to sponsor their topics will automatically appear on the details page, member home and submit topic page.
Displaying on Submit Topic
By default, the details of your prices are displayed on the 'submit topic' and 'sponsor purchase - topic type' templates. If you wish to display the info in other templates, such as the 'show topics in forum' template, you can do so with this code:
When someone sponsors a topic, their topic gets promoted and an expiration date for the promotion is set. Changes to sponsorship settings won't affect the expiration date. This means you're free to alter the price of a sponsorship level without affecting already-sponsored topics. Once the topic comes up for renewal, however, the owner will have to pay the new price to re-sponsor it.
The exception to his is pay-per-click sponsorship, which will deduct funds at the rate of the moment.
See Bulk Sponsorships to learn how to let people purchase topic type sponsorships in bulk with optional discounts.
To learn how to test paypal using paypal's sandbox with WSN, see this article.