I've used another service similar to listopt.com where I pay for leads and needless to say, it didn't work pretty well. Nothing wrong with the service, but if you do not know how to handle the new subscribers, don't expect them to be your customers - double opt in, triple opt in or not.
You will also come to find that some people, including veterans, don't laud this method as the top way of getting subscribers.
I got 450+ subscribers as a result of buying leads fast using similar method like what you've considered, but none of them bought my products. Either my approach was wrong (i.e. did not introduce myself, wrong content) or the subscribers are simply disturbed or not responsive.
I had one of my friends who is a subscriber to sites like you mentioned being bombarded (and spammed) by most newsletters she subscribed. On the same context, that can happen again. Which means, you can be labeled a spammer, or not taken seriously since your newsletter comes from that site.
I've not used listopt.com so I may be wrong on certain things stated above. However, it is NOT THE BEST method to get leads.
If you want to do list building, I suggest the normal way (which I am personally just beginning now on my own newsletter). Hard, slow but sure. Where you make errors, you can still correct them without paying out your money. You can't do that when you buy leads especially in numbers.