With the increasing number of rare items such as malachite shards, motif fragments, perfect roe there should ideally be a way for any player to sell the small number or even single rare they have found regardless of guild membership.

The basic idea is for each Thieves Guild Refuge to have a trader willing to sell your items which is accessible to any player. A black market trader if you will

To avoid this affecting the current Guild Traders there should be a limit in place. Something like 1 item per character or possibly account per week. The trader could charge a hefty fee such as 20%. This would keep guild membership the better choice for any player wanting to even slightly get into trading. Each trader's stock list should be separate to keep the market place feel of the current guild traders and to make it hard to for someone to try and monopolise the market.

If the limit is by character this could be tied into the legerdemain passive that increases the amount sold via a fence to raise the number of items per week.

This would hopefully allow players not into trading with only a few really marketable items currently sitting in banks to get them out into the marketplace and providing them with gold which they could then spend at the guild traders.

For example I have an Imperial crafter and have an Imperial motif sitting in my bank doing nothing for me. I would love to sell that and use the gold to buy the nirnhoned pieces I need for my crafter to research. What I do not have is lots of marketable items that I could put on a guild store week after week to make it worthwhile for either the guild or myself to join one.