I don't sell mine, but I time-shift with a small pile of batteries (about 10kWh) and it's pretty reasonable. I save about usd $30/month. It's basically a big ups that will pay for itself in ten years, and I get backup power.
I don't. The batteries will last longer than 10 years. The 10 year typical advertised lifetime of lifepo4 is to 80% capacity, and I'll just keep on using them.
The actual payoff calculation is a lot messier than that because you have to factor in the NPV of buying batteries vs. just throwing the money in the market, AND you have to be able to forecast that growth vs. growth in power prices. So the honest truth is I have no idea if it's going to be a net good investment vs other options.
Fortunately, I don't have to care, because I bought the batteries for UPS runtime, which I value independent of the time-shifting. The time-shifting is just a way to squeeze money out of an investment I already made. Had I been going for payoff, there are cheaper battery/inverter options out there with a sub-5y payoff.