Your toner shouldn't sting or dry out your skin like many harsh toxic chemical-filled ones at the store. This isn't an aftershave, after all! Your toner should also not leave your skin alkaline because your skin is naturally between a 5.5-5.9 pH. Instead, a great toner should leave your skin feeling soft, plump and glowy.
Your toner should also be professionally formulated for not only the pH balancing effect, but it should also have an effective preservative to ensure you aren't spraying harmful bacteria all over your face (a big concern in the natural beauty products industry)
The only time a toner shouldn't be professionally formulated is if you are mixing a fresh diy toner that uses an acidic ingredient like ACV for yourself regularly and keeping it refrigerated, and use it within a few days to a week. But who has the time for that hassle these days? You're better off saving time, and getting much better results with a professionally made stable one with loads of beneficial actives that you can't get OTC or find in your pantry.