Is a trade-in its own purchase order, or an adjustment on a sales order?
I would recommend modeling it as a sale with an adjustment for the trade-in, assuming that as a car dealer you accept trade-ins without a related sale of a car. 

Therefore within your system you re-use the purchase features, and can later report on metrics like how many sales are linked to trade-ins, what values of sales are linked to trade-ins, whether trade-ins boost sales, etc