Looking for an automated customer valuation tool: any recommendations?

Hi,

We buy products in from our customers and currently the process is they will send us an email with pictures of what they have, and we will then manually go through ebay to check last sold listings to then make them an offer.

The process is very manual and takes time so we would like to have a tool that can automate the process. Customers can just go on our site and type in what they have and we can set it to 'ebay last sold - 20% as an example.

I’ve had a look but struggled to find an app in the marketplace that could do this, even just giving a rough quote.

Does anyone know of a tool or app that would be along these lines before I build something bespoke?