I'm interested in selling my site. I was offered money for the domain and webspace. How do I let this person have it all without giving up all my usernames and passwords. Should I just give him the domain and the sql and let him upload it somewhere else?

transfer the domain to another account, or to another registrar, and let the buyer manage the domain himself....

I believe you have to go in to your domain control panel and change ownership there and to do that you need username and password. You might want to go in to your control panels for domain and webspace and change the passwords to something else. As for username I don't think its possible unless you get your host/registrar to help you with that.

For the domain, push it to another account internally with your registar and give him the new details. For the site, either nbackup the whole site via cpanel or get him to use a tool such as WHM Account Transfer to transfer the domain to hs server/account.

if you are selling the domain at a registrar that supports domain pushing or transfer between accounts just push it and and hand him over the data.