Logging in with an email address plus password is likely to be easier for the user most of the time (but see below). People might not want to broadcast their email addresses, though, so if this is a site where user names are displayed (ratings, seller/buyer names, social, etc), you'll also need to provide for a display name. Note that Amazon, eBay, and Google work this way.
One caveat, though: email addresses are not necessarily permanent, and you don't want to force someone to create a new account if it changes. If my address is email@example.com, that address isn't going anywhere and I'll use it everywhere I can; if, on the other hand, it's firstname.lastname@example.org (or email@example.com), that's a weak foundation on which to build an online identity.
Therefore, my suggestion is to have users create any user name they like, including using their email address for that purpose if they choose. Failing that, my second recommendation would be to use email address (plus display name) and provide an account-migration feature if the email address changes.