That can easily be perceived as violating the ToS and get you kicked off the marketplace. Big risk to take if that's where you're making most of your $.

I doubt it. I've seen this for products with 10,000+ reviews. It's totally mainstream. If Amazon had a problem with it, they would have cracked down already.

What can they say, that "20% off your cart" coupons are prohibited anywhere else the product is listed? That's not feasible. You see North Face doing the same thing with sports retailers. Their jackets never go on sale, but retailers make cart-wide promotions available so you can still get the discount.