I'll echo what's been said above, and add a bit of my own perspective.
My gun-related site has been around for four years. I've been doing search engine optimization (SEO) work for several years, including paying work. My site is now first-page for literally thousands of different search terms.
If I were charging myself for the SEO work I did on the site, the cost would have been between $3000 and $5000 a month.
Now the site is getting 14,000 to 17,000 visitors a day and roughly 1.5 million page views a month.
With all of that traffic, selling stuff off the site should be easy, right? Wrong. There are people who visit sites to buy, and people who visit sites for information. I've learned that the latter group make up the majority of my site's visitors.
So, I'm having to start almost from square one with Google AdWords and optimizing totally separate sales pages.
It's hard to compete on price. My cost on a set of Crimson Trace laser grips for a Glock is $152.96. I'm charging $189.95 with free shipping. After US Postal Service priority mail, and 1.7% in credit card fees, I make about $23. Even on $300+ items I'm still making $20 to $25. And there are a lot of outfits that are cheaper.
One of the biggest obstacles is trust. Outfits like Brownells, Impact Guns and others have been around long enough that people know them and trust them. Being the new kid on the block means it will take time to get known and trusted.
If you're serious about pursuing this, visit
WebMasterWorld.com and do a lot of reading.