IMHO ....

This (buying with the intent of returning) might work sometimes but, I suspect, you will eventually get caught and have problems. The merchants and/or credit card companies may actually pursue getting the points/miles back from your accounts. If they were to notice a pattern of this, it could also cause some of your accounts (credit card accounts as well as points/miles accounts) to be closed.

This also seems highly unethical .... intentionally doing this would be most commonly referred to as fraud (or close to it).

It should not be referred to as (or even associated with) "churning" which is getting credit cards, etc for the bonuses, closing them afterwards, and then repeating the process ....

I highly recommend against it. I suspect it won't be worth it in the long run.