When you purchase a van or other equipment that qualifies for tax relief, the cost of the asset is reduced - for tax purposes - by the amount of any capital allowance you claim.
Consequently, if you sell the asset at a later date you will need to compare the tax written down value (cost minus any capital allowances claimed) with the sales proceeds.
If the amount you receive on sale is higher than the tax written down value, then this profit will be added to your taxable income for the relevant tax period.
If the amount you receive is lower than the tax written down value, you can write off the difference against your related profits for the relevant tax period.
You don't need to physically sell an asset to trigger a disposal for tax purposes. You will also be considered to have disposed of an asset if you: give it away as a gift, transfer it to someone else, swap it for something else, get compensation for it - like an insurance payout if it’s been lost or destroyed, keep it, but no longer use it for your business or start to use it outside your business.