Hi Chris L
The action to take, depends on what was disposed of. Was it personal possessions, such as jewellery, paintings, antiques, coins and stamps, sets of things, eg matching vases or chessmen (this list is not exhaustive). If you disopose of of a personal possession for more than £6000, you may need to pay tax on it. Have a look at
Capital Gains Tax on personal possessions. If you sold stock you have purchased, with the intention to sell online, then this is trading. If your gross turnover exceeds £1000.00, then you will need to registed for self assessment as self employed.