The coder retains all copyrights to hack/application unless a contract exists.
Keep in mind that if you set limits or require that full copyrights be transferred to you .. you may have to pay more
It would be to your advantage if you encourage the sale (or sharing) of 'script' ...
SCENARIO:
If the coder decides to,
"sales it for dozens of people for a small $$") ... you will now have more people to find bugs, make requests, add new features, etc. In the long run, you and everyone will get a better product.
You see ... it's a win-win for everyone.
NOTE: No application is 100% bug-proof. Look at
vbulletin