ok, 100 points jump to mind instantly:
1. If you dont know where to source the product are you sure you know enough about the industry to compete?
2. The internet has different models - affiliate and drop-shipping are probably for you - BUT (and this is a big but) - you need to know how to get traffic. If you want to own a shoe store but dont know where to buy shoes then I'm guessing your jumping in with lots of enthusiasm but little know-how
3. I have a rule for web businesses - just cos you can, doesn't mean you have to. In other words, just cos you can build a shoe store, doesn't make it a good idea. There are thousands of companies in Ireland selling shoes. I know shoes was an example. Chances are that if there is no shoe sites (which there are) - then there is a reason
4. Billions of people - and I've met most of them - have this idea (wtf) - "Build it and they will come". This doesn't happen. People never believe me. I have their analytics to prove it. If I look into my crystal ball here's what I see "When you see the first design for your new site you'll be so bowled over by it that you won't believe how millions of people will just happen upon it and they'll refer it to their friends and then within 8 hours of going live you'll being making offers for Google..." no seriously, you probably will argue that if it just "ticks over, maybe it'll generate a couple of grand" ...yip, it's that easy, just tip away. Ain't gonna happen
5. There are very few sites that dont have big advertising money - or are not run by SEO's/online marketers - that just tip away and make money. Ask any web business owner here - it's a business - put nothing it, getting nothing back. Put loads in, you might do ok.
But dont listen to me. Go build your site. If you have customers, you'll find a supplier. HAve fun. I will say this - DO setup a website, do learn, do fret about bounce rates, visitor traffic, Google SERP's. Do this before you spend big money. Then go and do it again but do it right. Best honest advice I can gives ya