Apparently in China people don’t treat tomato as a vegetable but as a fruit (which actually is, from biological point of view). If you want to buy tomatoes you should go to a fruit shop. As a form of dessert you can find them in a bakery, decorating cakes (so don’t take them for strawberries by mistake). They are also sold by street vendors covered in caramelized sugar. In other words, sweet tomatoes wherever you go.
How does it taste? Peculiar. Neither bad nor good. Just odd. If you wish to experiment you don’t have to go to China for that. You can easily do this at home sprinkling tomato slice with sugar.
Have a nice fun!