It can be pretty much anyone. It could be me. It could be someone's personal financial manager. It could be a mutual fund manager or an automated program designed to trade according to specific rules. It could be your neighbor's grandmother. It could be the company buying back stock, but this doesn't happen too often - usually only when the company is in a really strong financial position or they want to prevent buyout, or in the case of something like Twitter they did get bought out and are trying to turn from a publicly traded company to a completely private one.

If you are trading at market values, which is about what people are willing to pay for the stock or sell it for, then you'll have a much easier time actually selling it. If you try to sell something that is currently trading at $10 for $100 you'll see that the order probably is not going to go through unless someone is desperately trying to buy as much of that stock as possible and put in a very high bid for a lot of them.