This is something that I have been looking into also, and am just starting to get into advertising for specific product niches. I looked at Buy Sell Ads, and comparably speaking, they are quite expensive. The best advice anyone can offer is to due your due dilligence and take the time to research your specific niche before jumping right in. Banner ads are a great option, but only if the ROI is worth it. This can be quite tricky because you really don't know if it'll be worth it or not until you try it out, but you have to put your product in front of people who will give you the best chance for purchases, signups, or whatever specificaly you're looking for.
Adwords and PPC are always reliable, can be quite expensive as well, but allows you to laser target your audience right down to the fact there typically there to do exactly what you want. Say for instance you're targeting "buy pink and white shoes", then this set of keywords are targeted and you will have a higher chance of finding someone specificaly looking to "buy pink and white shoes"