i recently started a website selling mobile phone contracts as an affiliate through the likes of carphone warehouse, mobiles.co.uk, e2save and so on.
First off do you have any idea how to design a website using dreamweaver or other software? if not you're in for a rude awakening. Depending what browser you are using you can usually view the page source (what makes the website look and act the way it does) if you're using firefox right click and click on view page source. If that stuff doesnt make sense to you then its gonna be a long and hard road ahead of you. Not impossible cause i started from scratch and had a working website within a month. If you dont want to do the design side of it you can use
www.1and1.co.uk they let you use there pre built templates and i think they start from around 6.99 a month with your first month free but please believe me when i say their templates will NEVER allow you to build a site that will compete with ebay/gumtree or the likes. they are designed for small business owners wanting to promote their services.
Last but not least is the amount of time involved in getting your website functional. If you have 3 or 4 people under you then its feasable. I eventually had to give my site up simply becuase of the amount of time involved keeping it up to date.
Someone with masses of knowledge could keep it going so it all comes down to how much you know about doing this.
Dont give up dude if it wasnt for people like you wanting to do this we wouldnt have sites like ebay, gumtree and golfgtiforum.co.uk
learn what needs to be learned and the first place to start is go to google and type in "how to build a website" its where i started and had my site within a month. one page will lead you to another with more info.
Good luck