Allow pricing NFTs with the use of available offers

[0xmichalis]

Abstract

Currently rotki uses the floor price of a collection to approximate the price of an NFT. This is fine for big collections with high volumes as usually the spread between asks and offers is narrow but can be deceptive for low volume collections as it gives the impression that price is what someone can get by selling their NFT which is not the case.

Motivation

For low volume NFTs (fine art, and other low velocity use cases), floor prices introduce ambiguity to the value of a portfolio. Currently, it is possible to provide custom prices to NFTs but it is cumbersome to manage in a big collection.

Specification

I think the simplest solution would be to allow users to switch prices between floor (current behavior) and available offers, maybe with the use of a toggle. For maximum customizability, it would be great to be able to toggle this per NFT but a global toggle would be a good start.