You would have several invoices, similar to the guide I linked to above.
A sales invoices for the sale of the stock vehicle (£5,000).
A purchase invoice for the purchase of the vehicle (in this case, £1,000).
A sales invoice for the sale of the vehicle (you mentioned the same price, so this would also be £1,000)
Overall, the effect of the above is +£5,000. You would use the actual values you buy and sell the vehicles for. Even though you sold the vehicle for £1,000, which happens to be the same price you purchased it at, you would still need the records of £1,000 to show the actual transaction.