If the tire were to be used on the rear of the car, the worst that would happen is the ABS light would come on (and possibly the low tire indicator, if so equipped). If it were on the front end (since the Passat is a front wheel drive vehicle), you run the risk of damaging the transaxle (the differential in the transaxle to be specific). When running tires of different sizes on the drive axle, you are forcing the transaxle to behave as if it is constantly going around a corner. This creates a lot of wear on limited slip parts as well as creates a lot of heat. Since it is collocated with the transmission itself, the added heat creates an issue for the transmission portion of the transaxle as well. Heat is the biggest killer of automatic transmissions.
The difference between the two tires you mention is somewhere to this difference:
- 217/45R17 = 24.6" overall diameter = 77.3" circumference
- 217/65R17 = 28.0" overall diameter = 87.9" circumference
Please note this is over 10" difference in circumference ... that isn't just a little difference, that's a HUGE difference. If you'd like to play around with how this all figures out, please take a look at this site. Remember, though, this is just a rough estimate as while the numbers look the same, it doesn't mean that one manufacturer's claim to width or aspect ratio is going to be exactly the same as another's.
I have two suggestions for you. First, continue to look on Criagslist. You'll find what you are looking for there. It doesn't sound like you are in a huge hurry to get this done (you don't need it today), so be patient and get what you need. Secondly, see if there is a used tire store around which might have a tire for your vehicle. At least in my area, there are several which charge a set amount for the tire and for mounting/balancing and are very reasonable.