I know people (person) that makes good money on ebay. A lot more than £300 per week as well. It's all down to your product and how much people want it the same as any business.
I run an online business, unfortunately my products are not suited to ebay but I wish they were, you save a fortune on advertising, internet hosting fees etc etc etc.
If I could have my time again ebay is the way I would go. Get yourself a product buy some from the shops put them on ebay at the same price slightly less, if they sell then source a cheaper way of getting those products in bulk. If not get another product and try that.