As they will either have to sell the home on the open market or sell to an investor, they will probably offer you substantially less than you might gain on the open market. In the past, these kinds of companies specialized in distressed properties, estates, etc where the owner wanted to cash out quickly and wasn't concerned with getting top dollar.
You can certainly talk to their representative about what they would offer for your home. But before signing with them (signing ANYTHING) talk to two or three local real estate agents and ask them what you could get for your home on the open market. And also ask them to provide a rough estimate of what you will NET from the transaction after real estate fees, taxes, etc as this will give you a clearer comparison between the offer you get from 1 800 sell and what you would get selling the traditional way.