They may be relying on the type of people who really, really, really need to have a new car every X months/years.
These people may feel that they will buy without a trade-in and then sell on their old car themselves instead of through the garage.
The dealer could easily pitch this as "Arrrra, sure we couldn't give you a good price on the trade-in at all so you would be better off selling it yourself and getting more than we could allow." Looked at that way, some people might convince themselves that it makes sense and that they somehow will be able to get a good price on the second hand market when the dealer can't. Dealer sells car, doesn't have to offload trade-in vehicle, smiles.
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