Yesterday, 11:30 PM
The biggest challenge is to use image processing techniques to identify the item, if the user adds some data along with the picture, it makes it a bit easier.
To get the value, you would need to identify some data points, a big one is sale price for the same or similar item at retail, or via ebay, craigslist, amazon, google shop, etc. Most of these platforms have search APIs you can use to find pricing. Some APIs are not free, but should give you enough for free (in demo/developer mode) to know if it is worth paying more for better functionality.