Do all the things you normally would when selling a product to someone you haven't met, for example on eBay:
- check references if possible
- Use an escrow service or ask to be paid before handing over the site
From a sys admin perspective:
- Change all passwords and hardcoded personal info
- Remove all trust relationships with your own servers
- Announce the change of ownership on the site
And contractually:
- Ensure the contract of sale passes all liability on to the new owners
- Specifically state that there is no support or warranty
(caveat: I am not a lawyer, but these things seem to be good sense)