i have 1000guppies in 100gallon tank is this ok

yeah should be fine

Not really. They will die off and the stocking will come back to an equilibrium with the tank again. Expect to lose around 800-900+ guppies in the next year since the tank cannot support the stocking level you reference. These things have a way of balancing where the overpopulation will die off until the tank can support the bioload again.

You could sell most of them and make a little money though. Even at $0.50 or a $1 each you should see some nice returns!

That is way overstocked. Get a huge bubbler and do a water change every day and you might be able to pull it off but really you should consider getting rid of some. Usually you can sell them to pet stores. They typically prefer locally bred guppies to having to order them anyway. Another thing you could think about is buying something like an Oscar or a Jack Dempsey if you have another tank. You would have as many free feeder fish as you wanted if you are okay with feeding your pets to another pet...