While I suggest reading through my answer on the used car market (https://www.preplounge.com/en/consulting-forum/size-of-the-second-hand-car-market-in-the-us-14555) there is a key difference here. While virtually every car will be sold at some point as an used car, this doesn't apply to furniture. Also, a car has a limit to its useful life, furniture doesn't. On top of that, many people give away used furniture (to family, friends).
In short, you cannot cannot simply assume that a table will last 30 years and will be sold every 10. That is just irrealistic.
I suggest you approach it from a slightly different angle. You need to consider more carefully WHO is purchasing the used furniture and why / when.
In my experience, you have two segments. 1) Young people, regardless of income level (with a few exceptions), as they leave their parents homes, go to college, professionals that move city for their first job, and similar siutation. This is a one time situation. 2) Low/low-middle class people. These will trade used furniture multiple times. [be careful not to double count this situations].
On the business side, you have offices, restaurants, hotels, retail, services, etc. Only those that are HIGH ROTATION (i.e., businesses usually don't last long, so they close and need to sell their furniture to someone who's entering the business) are likely to be relevant.
Meaning that restaurants is your #1 customer. Everyone else is not very relevant (retail only needs some shelves, small offices is mostly basic tables and chairs that don't last that long, hotels may refurbish but usually try to maintain as long as possible…).
So I would just assume that restaurants are 80% of the market and use it as a proxy.
Finally, yes, you'll need to consider how much furniture they have, what's the value of new, and what's the discount they'll have. I would do simple assumptions here, i.e., something around that the furniture is only sold once and at e.g. 30% of their new value.
This is not a full answer, but hope this helps.