Yes it legal to sell addresses, but don't waste your money. 999.9% of the time these list are old outdated lists. In addition in order to sell to the addresses they have to be 'opt-in' addresses. In other words the owner of that address has to give you permission to mail to him. Otherwise it's considered spamming and if someone complains you can get tossed off by your mail provider, or hosting supplier (get your website shut down).
Even if the list were valid, most often they are just groups of emails that someone has gathered one way or another..In order to sell to a list you need a 'targeted' list. That is to say that if you are trying to sell video games, you want a list of people who are interested in video games. Most lists will have all kinds of addresses from people who were buying all sorts of things. You don't want to waste your money and time sending emails to people who are interested in baby accessories and such and on most of these types of list that is what you are going to receive, that is, a collection of addresses from people whose addresses were captured from buying all sorts of goods..
An actual valid opt-in targeted list that is up to date can be quite expensive.
If you are wanting a list to sell to, I would suggest trying to build your own list by offering things like an opt-in newletter about the goods or services that you are trying to sell...or when you sell an item from your site or auction include an opt in selection box on your invoice or follow up email and try to build your list that way and you will be money and time ahead...