As more of anything is out there, more will be for sale used, but how badly will anyone want them? They degrade over time and installing is a huge cost. How much are you willing to pay for mismatched used solar panels from a 15 year old system?
Might be interesting for a "do it yourself" project, from someone after retirement who has enough land space. But only if the price is low enough.
New panels without installation goes for what now ? Maybe $1 or $2 per peek Watts ? I would say 10% to 20% of the new panel price would be interesting.
More like 60 cents. Installing/connecting the panels generally costs far more than buying them even if it's a DIY job (which, seriously, it shouldn't be. You could get someone killed).
Also the modern panels tend to produce more power per square foot than panels made just a year or two ago. And they often come with a warranty between 25 and 40 years which you won't get secondhand.
I could see a certified refurbished market someday but secondhand panels are hard to sell. When I've bought them I've paid about ten bucks or so (and that was just for hobby projects - the ones on the roof of my house? Bought those ones new).