ive noticed this alot, not with cars, but in general.
i sell a few things through gumtree, and ONLY post my phone number, i untick the option to accept emails.
what ive noticed is that people who text and email to buy things (with the exception of ebay's messaging system) just want to test the water, to see if you will budge on price without having to talk to the person they are trying to buy off. its alot easier to mug someone off by offering half the price through text than it is to say it over the phone, or to their face.
another thing ive noticed is that people who ask for you to call them are often time wasters, who unlike the above try to hustle you on your dime, or people who cant afford to waste their credit, so often cant afford to pay what you want for your item.