It's not that old that's why it's not that valuable if you sell it. But you could still try and look for a collector and maybe they could buy it for a small price. Or you could give it to me. Just kidding. Why not keep it? Put it on a time capsule. After how many years you could get it and remember your memories with it.
Depending what country you're based in, there are options - where I live there's a site called Gazelle that accepts phones. Where are you based? It might make it easier for me to offer suggestions if I know what country you're in.
I wish you lived at the place whre I live and theft is like a very common thing. We always keep our oldest phones as a means of jumpstarting our lives after a phone muggery. That is before you accumulate enough cash to buy another one.
Amazing! I have seen some people who were looking for those kind of iphone which they consider vintage phone already. You may take care of it becaise we can never tell if one day, it may cost a fortune.
I would keep it until it becomes antique, or antique enough for it to be worth it's original price again. Although, in the event that it breaks or doesn't work anymore, scrap it, either by selling it for whatever amount you can or dismantling it and using the part for something else. The second option works better for people that love tinkering.