There are legitimate and illegitimate reasons why someone might ask for your email address on Craigslist. If you’re trying to buy something, they will need a way to respond to your offer, and the site pretty much gives up your email address by default. Scams, spam advertising, and other malice also involve your email.
Most people who cover their license plates online probably do it to try to hide a little bit of personal information. They might be concerned with what a person could learn and do with just a license plate number. It’s not something you definitely need to do yourself, but people who do are often worried about privacy.
Most likely, the item has already been sold, and the seller hasn’t found the time to remove the listing yet, and they have deemed replying to you to be more effort than it is worth. It’s also possible that they never saw your message because of spam filters or other issues. They might also have changed their mind.
Using Venmo for financial exchanges via Craigslist comes with inherent risks that are not addressed by the app’s design. While Venmo does offer good security for users, there are scams and exploits that are more common on a site like Craigslist. Venmo suggests that you don’t use the app in this way.
This is mostly done to get around software that is looking at phone numbers on the website.
Craigslist sometimes uses software that blocks or removes phone numbers from listings for a variety of reasons. There are also third-party bots that crawl the site looking for phone numbers and contact information.