If I buy something privately and then sell it on at a profit (as a one off) am I breaking the law by not declaring it?
If it’s a one off then it’s unlikely it will be taxed as part of trading income, so your only possible exposure is to capital gains tax.
As said it depends entirely on what the “something” is. Give us a clue!
Generally the following types of assets which can increase in value are liable to capital gains tax:-
Shares in a company.
Land and buildings (apart from your own residence).
Higher value jewellery, paintings, antiques and other personal effects.
So if you’ve just sold that lovely Picasso hanging in your hallway then yes you’re required to declare any taxable gain.
The following types of assets are exempt from capital gains tax:-
Your private car.
Your private residence.
Jewellery, paintings, antiques and other personal effects that are individually worth £6,000 or less.
CGT allowance as an individual is £8000 per year.
The annual exempt amount is currently £9,200.
If the profit you made on the sale is more than this you might be liable to capital gains tax.