The

color

option for players offering quests seems like something for wishlist (makes me think of suggesting advisors get a color..

.).
As far as your general idea, yes, i already know of players & guilds who do offer quests IN character. Usually they are simply a find this thing or person, and announce the reward (some consider events to be like quests but that's a different), and are generally limited time offers. And usually generous or at the least somewhat interesting.
As far as being scammed, unless the player giving the quest decides they want to rp like an npc & only say "Do you understand what you are saying? I do not." or such when you ask about the reward... newer players know well enough after an npc quest or 2 that often what the quest asked of them is worth more than they are given upon giving that item.
and it is already such that when giving an item to an npc you only see yourself giving the item untill you accept the trade-- but with players the trade window opens both ways, so you can see what is being offered before clicking the button to accept the trade. Unless you don't know what something is worth it gets a little hard fo them to scam you (again, unless they claim they are rping like an npc & will only reward quest after being GIVEN an item... but who has patience fo someone rping like an npc anyway?!)
Besides, while we all value our items; experience is the most valuable commodity in a roleplay. It can get frustrating trying to find more quests to do --more player quests means more to do. It's certainly something i like the idea of... ( And i hope not to regret not having searched "player quests" & "quests" before replying...)