Just do a search on it, and you will get huge list of names. Beneficial? The odds are high against you. Many who give out their address do so with a throw-away address, like a Yahoo or Gmail account. After a short while they no longer use the address. You just bought a dead address. On top of that, even if you do get good addresses, most stop reading mail after the second or third mail sent to them. If that much. Then you have the very first obstacle, which is getting them to even open your mail to even see your sales pitch! The odds are that more money will be spent on these lists than you can bring in. Example: It will literally take hundreds upon hundreds of e-mails sent to make even 1 sale. How much will that many names cost you? And, how much is the product you're selling? Think about it.
The best way to do it is to set up a squeeze page to capture email addresses and then use an autoresponder service like Aweber or Getresponse or iContact to send them emails. In order to get people to opt in and actually OPEN your emails you have to continually provide them with something of value to them. It's no different than having a blog or any other website. If people know that it's usually worth their time to open your emails, they'll do so. If they think all you're doing is sending them sales pitch after sales pitch, they'll ignore you. What copper12 is referring to is the response that people who DON'T provide any value to their list get. After awhile people ignore them because there's nothing but ads and sales pitches. You can still promote products to your subscribers, you just have to do it while you're providing value to them.
Hey, I get 30-60 opt-in each day just by writing articles. This could easily be 7,000 to 15,000 opt-ins in a year just by writing a few articles each day. Try it, you might be surprised by the results you get.