It would be almost impossible to automate looking for the signature markup on some user's profile because there's no database of them all.
Wouldn't be hard at all for BPIP or LoyceV to scrape signatures when they scrape the profile. Duplicate signatures can be detected instantly.
Or, Theymos could create a database of paid signatures, and you'd have to select one if you wanted a sig. How many people notice signatures anyway?