Depends on how many bikes your selling, if its an income stream for you then yes you better report, but if it's one a bike a year or so then no. There's an old saying: "what they don't know won't hurt you". You won't get audited for this anyways, who's going to report you? Even if you got the bike out of the dumpster, put $20 into it and sold for $1000, the IRS (I know you're in the UK and you don't have the IRS there, just go with it) won't care nor will they know about it anyways. Of course if you volunteer the information then you might have to pay taxes on it, depending on the auditor, some will say you better pay, other will say just forget it...unless you're selling 4 bikes a month every month and making that kind of profit as in my example. Of course if fixing up and selling bikes is your income then you need to be keeping all receipts for expenses related to that because as a business those will be tax deductible expenses as might be the work space your using out of your home. But it sounds like to me you more or less break close to even, if so I wouldn't bother reporting it, it would would be more headache than it's worth, and again who's going to report you?