so my feeling is that the best way to do this is send your eDM via someone else's list who has their subscribers trust (like an affiliate marketer would do)
i'd advise against this, if you haven't gained the list YOURSELF then you will trash the trust of the person who gave it to you with their clients.
there is another thread on this where i subscribed to a newsletter and then got emails from the guys wifes' NEW business which didn't make me happy because the ONLY way he could have got THAT email address was via that newsletter
there is another thread on dnt about scraping YP data but not sure if we ended up getting email address processed into excel or CSV
as for legal, i don't think its illegal once it is in the public arena eg: YP, business card, paper advert etc as long as you make sure you have an unsubscribe then you should be fine.
i also researched buying lists and it was quite expensive i felt with now guarantees and often MANYYY duplicates.
tim