The APK has expired
You missed the date, or the car has been standing still. This often happens when someone has been abroad for a while or stopped using the car over winter.
The APK is the Dutch annual roadworthiness test. Without a valid one, the car may not be driven on public roads, except straight to a garage for a test. It can still be sold. Paying for a test, and then for whatever it fails on, is rarely worth it when the car is going anyway.
You missed the date, or the car has been standing still. This often happens when someone has been abroad for a while or stopped using the car over winter.
The garage gave you a list of what needs fixing before it can pass. You may sell the car in that state.
Rust in the wrong places, worn suspension, an exhaust near the end of its life. There is no need to pay for a test to confirm it.
If the registration is suspended (geschorst), the car does not need a valid APK, but it may not be driven or parked on a public road. Tell us, because it changes how we collect the car.

The more of this you include, the more accurate the offer, and the less likely it is to change at the handover.
Only for a short time. The RDW allows a car without a valid APK to stand on a public road for up to two months after the expiry date. After that it must pass the APK or be suspended, or you risk a fine.
Suspending takes the car off the road in the RDW register. You pay no road tax and need no insurance or APK while it is suspended, but the car may not be driven or parked on a public road. You can sell a suspended car.
No. Tell us when it expired or what it failed on, and we take that into account in the offer.
Before you deregister from your municipality (gemeente), if you can. Selling while you still live here is simpler, and it stops the road tax and insurance on a car you no longer use.
Enter your number plate and mileage. Asking is free and you are not committing to anything.