If you only have a small amount of start-up, you'd be best working through eBay and Paypal.
The above posts are right, any business is tied in with a whole boatload of legal issues, most designed to do the seller over if the need comes about and less designed to protect you from buyers. You also need a damned good idea to stand out and simply starting up a website alone can be a huge drain on resources and such.
If you work through eBay, you can gauge your chosen market, see your competition directly and you have a stable platform on which to list things; essentially all you're doing is buying and selling, you will have no other set up costs. It also allows for an easy transition over to your own website at some point.
eBay also makes life easier when you look at the legalities. Paypal is designed to protect everyone and provided you work to their rules and guidelines you're pretty well protected, you also avoid all the legal mumbo jumbo which requires you to fill out tax declarations and VAT forms, well, mostly. Everything about the system they use is designed to streamline things for you...
I personally set up a business properly for my photography, I looked after it for a year and made about £180, I then decided to give up calling myself a business and worked personal jobs based on contributions from people who thought I did a good job, this made me roughly £150 a month, much better.
Start small, if you latch onto something and do well, you can expand from there...
Any legal queries, feel free to ask.