Its cold out there man! anyway to answer your question. Best place to sell it is to a friend. The only way to know how much is it worth is to find your components USED on a website. Search for the exact cpu, exact gpu, a generic motherboard with the same socket as yours, case, exact PSU (or generic one if its a cheap model) and do a total.
If its still new and you have it under warranty then cut about 30% of what you paid and sell it at that price WITH the remaining warranty papers.
The best place to sell it is to a friend, or a friend of a friend as it will sell faster than on a website. Regarding your "sell to a store" question - Don't do it! they will rip you off and might not even want to sell it unless you buy a new one from them.