What is cost basis?
Everything an item really cost you to get ready to sell, not just the price you paid.
Cost basis is the purchase price plus the costs that belong to that item: travel to the car boot, cleaning, repairs, a new zip. It is the number profit and ROI are worked out from.
Example: a jacket bought for £18.00, £2.50 of cleaning and £1.50 of travel has a cost basis of £22.00. Leave the extras out and every item looks more profitable than it is. Keep receipts too: the tax on your selling is on profit, so the costs you can show bring the bill down.