This is very true however, you need to have a decent turnover to make any money and you need to pay out more than anyone on PAYE for any "business" things. As a very rough guess, I would think that you need to have around £9,000 clear income to cover the running of a small business. By income I mean that if you buy a bike for £500, spend £100 on it and it costs another £100 for fees, you petrol etc you are into that bike for £700. You sell it for £1000 so your income on that bike would be £300. This obviously does not include any tax, NI or the cost of running a business. so you would need to turn over 30 bikes a year just to pay for running the business. This doesn't include any salary or drawings for you.
Your site also is breaking at least 2 current laws;
No cookie information
No company details
This should be your first plan. Sites are trawled and you may get a visit from HMRC and you will end up in a pile of it.
Sorry if this sounds negative but this is the reality.