Can you use tin cans to start vegetables in?

It will work just fine. I only give mine morning sun as it is hot where I live and the tin heats up quickly and then it is too hot for the seeds.

Make sure there are drainage holes.

Yes you can use tin cans as in the food store kind!! drill holes in the bottom or use a nail and a hammer two or three holes will do and a couple of small rock or a few pea stones and put in dirt and the seed you can then put the cans on a old plate or what you have around the house to place water in and the soil will suck up the water like a straw!

Did you ever eat caned veggies and lived?

I don't see why not the cans will need drainage holes, why not paint them in bright colours. Do think about how hot the plant roots will get though as you will know that metal conducts heat and could get too hot for the plants. Place them in a semishaded place that gets some sun. Always water in the mornings as it could get really cold at night depending on where you live as this could freeze the water if watered in the evening. Good luck.

you can also use paper or plastic cups, egg shells, egg cartons, the plastic trays food comes in actually they work great for starting seeds especially the type with the attached tops they work as mini greenhouses just make sure you punch drainage holes in all of them and make sure they are all CLEAN