For GovDeals, the buyer can do it by logging into their account -- steps below -- or the Customer Support Representative can do it via Entity Manager.

1) Login.
2) Hover over your name.
2) Select “My Account”.
3) Scroll down and select “Edit”.
4) Change your email address.
5) Select “Update”.

For AllSurplus and GoDove, the buyer needs to contact Customer Support.  The change is then done via Entity Manager.