I just saw the sticky option in WSO and other Sub forums. I really suggest to think of a queue, rather than direct buy option. Whenever you push the button, it has to added to queue and show you the thread live date and you can pay for it.
At this stage, you don't know when the sticky is going to expire. Only one who knows this is sticky thread holder. so, he can keep his thread at top.
It is just a magic button currently. :p