Personally i wouldn't buy a dented iPhone, although a cover would probably hide it but people worry about these things and what further damage the drop may have caused especially if you're selling online where little truth is said about used items.
A jailbreak is something people could wait for but the dent would still need to be fixed, i think you're better off getting a new replacement while you can, not everyone is looking for a used iPhone 5 has it's jailbroken status as the decisive factor to make the purchase.