Hello, I have an idea for a site...that would essentially promote a lot of different items...say sporting goods.
Will I be able to be successful targeting a wide arrangement of items on one site?
You can, but the more you tackle the longer it may take you to see results.
Think about it in the grander scale.
You have big stores like Walmart which have everything (like your sporting goods), you have medium stores that are "sporting goods stores" and then you have more specialized stores that might just sell lets say "Baseball stuff"
Now, even within that baseball store there are going to be a lot of bats, balls, golves, diamonds, etc.
If you go too wide with your offerings you are going to have to tackle more and more keyword phrases to get traffic to your site.
If you choose something very specific you can get a better picture of your competition as well as what you need to rank for.
The list shortens greatly.
Unless you are able to brand yourself within a specific region, trying to brand yourself on the web is going to be a lot of work - unless you have beaucoup bucks behind you.