I've never seen a trade-in value inflated to reduce cash.  I've just seen it by dealers who then inflate the purchase price by an equal amount.  

I haven't had any clients trading in business vehicles in the past few years, so I had forgotten about this TCJA tax increase.  But it's probably not that onerous, with Section 179 or accelerated depreciation available on most vehicles. Or are they?  And be sure to point out to the clients that they should have waited until EV tax credits were once again available, as long as the batteries are made with Chilean lithium.  (Chile is rewriting its Constitution under a new Socialist government.  We want to encourage that.)