What is the legality in the UK/USA from taking an e-mail address from e.g. eBay or Craigslist and sending an e-mail to say "Try this site, it's like eBay/Craigslist but it's free to advertise ..." Thanks, Khalid.
It's against the law and you could face criminal prosecution, but more likely, hefty civil penalties.
Sending this type of unsolicited email is spam. You can research "SPAM" laws in the UK and US. Also, research the CAN-SPAM Act. "Harvesting" email addresses is also against ebay's policy.