Is it illegal to see beautiful pictures (such as oil paintings and photos of foreign artists) on the Internet and make postcards for sale in Taobao? How to do well

It's definitely illegal to sell other people's things. It's simple and clear.

If you have to do this, you can only hope that people will not find out or will not pursue it after they find out.

If it is a small business, it won't make much money, and it is estimated that people are not interested in litigation.