Keep in mind that many shopping carts you see on the streets have actually been stolen from local grocery stores. If your cart has identification on it, you can call the store and they may send someone to come pick it up.
If the cart is completely trashed... then it may not be worth much for the store to recover... thus good for projects (keeping in mind that it may have begun as stolen property).
Oh, one other thing, be careful with welding chrome. You might look up more information about chrome.
If you are comfortable with welding and fabrication, you may choose to build your own trike frame, then just use the shopping cart basket.