The main difference is concentration and professional application. Sure, you can buy retail acids and retinoids but they're formulated at strengths that are safe for unsupervised use, which means they're honestly not as potent as what professionals use. A professional-grade peel has much higher concentrations of active ingredients and the professional knows exactly how long to leave it on your skin for optimal results without causing damage. They're also trained to recognize if your skin is reacting badly and can neutralize the peel immediately. Trying to DIY strong peels at home is risky because you might not apply it evenly, might leave it on too long, or might not realize your skin is having a reaction until it's too late. The results from professional peels are just noticeably better and faster.