I see not a lot of business minds in here... you don't necessarily need a niche product (although this helps early on), you do need good demand for whatever product you choose to try and sell.
I did pretty much exactly what you're trying to do... built my own internet business. It was pretty successful monetarily, but I did not particularly enjoy the work. As to your questions:
1 - To not get "raped" you have to have some knowledge of the process / work needed to complete something. In my case, I built my own websites to avoid costs, but I'm sure a professionally built page would have been better (and likely worth the investment). You have to decide now whether or not you are OK with losing money, because this will happen.
2 - I paid for a shopping cart as they handled all the legal requirements for storing CC information and made setting up product sales and tracking data much, much easier. Cost me about $35/mo if I remember correctly (1ShoppingCart, although looks like their fees have changed drastically, I never paid per-transaction like they have now) and I thought it was worth every penny. Otherwise, you'll have to either design your own system or buy one then get certified to be allowed to make sales.
Also related to this, you'll need to set up a merchant account and link this information to whatever sales system you set up. This is how you accept credit card payments. These merchant account companies are all scummy so I do not have a personal recommendation. In my case, I joined one through a guy I knew because due to his sales volume & recommendation, I received lower fees than pretty much anywhere else. I had to borrow from his volume though ($1mil/yr + ).
Also one last note here, in addition it is worth while accepting PayPal even though the fees are higher. Probably 20-30% of my sales were PayPal, people just trust using it when paying at a new / unknown site. You will either need to set up a business account with PayPal or convert your current personal one (which is what I did, huge mistake if you ask me - I would just set up a 2nd PayPal account for this one).
3 - As far as shipping, I exclusively used USPS as it was very easily the cheapest of the 3 for inside the US. They had a very good system (I thought). Initially I was going to set up using all 3 major companies, but you'll quickly learn that's a huge hassle as you need to keep on top of their rates to make sure you're not losing money to shipping. If you get big enough it will likely be worth the effort. I mainly chose USPS because I could advertise 2-3 day deliveries (I exclusively used Priority unless the buyer forked out for overnight delivery).
But yeah, you'll be spending a lot of money up front. At one point I was spending about $3k/month on advertising but it ended up being worth it as I saw my sales increase month over month over month. You can't be afraid to spend / invest.
Good luck!