Your income you receive from hairdressing is taxed because you are running a trading business with a view to making a profit. Selling goods at car boot sales is potentially also trading income, however each case must be considered on its own merits.
For somebody like yourself who has only sold their own unwanted second hand goods on a couple of occassions, HM Revenue & Customs would not deem there to be a trading activity and therefore the income received would not need to be declared on a self assessment tax return. Likewise, because the income is not declared, relief for any associated expenses incurred (such as pitch fees and petrol/motor costs) cannot be claimed.
On the other hand if somebody regularly attends car boot sales (for instance every fortnight) with a view to making profits, HM Revenue & Customs will deem them t be trading and they would need to be registered as self-employed for tax and National Insurance purposes.
If you are uncertain whether or not you will be deemed to be trading, please speak to your local TaxAssist Accountant who will be able to advise you based on yoru specific circumstances.