Super Mario Run – Can Buildings Be Sold and How?

No, when you purchase an item the amount of toads that you have is unaffected. The only thing that you need to buy these items are the coins collected in the levels. But when you have more toads, your town will level up and unlock more items to buy. Remember though, all of the buildings are just decorative, and only the special items have a purpose.

You can however, lose the toads that you already have. If you enter a Toad Rally event and are beaten, then the toads that your opponent wins will come out of the amount of toads that you have. So you'll be looking to be losing about 25 toads when you lose.

I've marked this as a community wiki post, because I think this will take multiple data points.

I've played on a mix of cellular and wifi since launch day. On cellular I've mostly played Toad Rally, which would naturally use much more network bandwidth. I've also done some shopping and read the notices/alerts several times. I've probably played about 3-4 hours on cellular (this is a total guess) and I've used 26mb of data, which includes initially setting it up and a partial DLC download before switching to wifi.