Kelly Blue Book for second hand electronics

I was recently trying to buy a new phone on eBay and I was trying to figure out what was a good price to place a bid at. I thought about it and what if there was a kelly blue book type website but for electronics on the second hand market. iPads / Laptops/ GPUs etc. It would aggregate prices from various sites and track the average. Does anyone think this is a useful site or does something like this exist already that I’m not aware of? Thanks, GP