Membership is FREE, giving all registered users unlimited access to every DNForum feature, resource, and tool! Optional membership upgrades unlock exclusive benefits like profile signatures with links, banner placements, appearances in the weekly newsletter, and much more - customized to your membership level!
You should either "push" (transfer) the domain to a new account in the name of the purchaser, or they should request a transfer to a different registrar.
If you give him access to your account, it means you can't use it for any other domains and you will possibly be giving them sensitive information about you.