I've been in the advertising business for a while now.
My opinion (of course its only my opinion) is simple.
Pending on the time you have, and cash (if available)
Use every available source.
Guaranteed visitors sites are good but you tend to have to get
say 300 people there to make one sale, BUT its free.
Submit to all the search engines..again free
Multi submit to Classifieds....Free
get my drift?
If you use every possible means, you've covered every corner.
If you also use a program like roibot.com
You can then see where your sales come from and you will then
know whats best for yourself.
Obviously the easy way is to pay for your visitors...but there
are a lot of scams around..Just be aware..