An old (2012) article from the BBC about recovery of taxes from those trading on online sites. It raises an interesting question: At what point did you switch from selling surplus items to actively trading? The key event seems to be when you actively purchased items to sell at a profit.
Have you declared this to your local tax authority? You are probably aware of how much power the ‘taxman’ has and you would be wise to make sure that you have covered a tax liability and declared your activities.