If you are serious about selling you would be selling now. There's really not much else to it. I am not someone who pays any attention to the specifics of the trading card $ market so all the following is basic economics. Take it with a grain of salt if you want.
Has the market peaked? Who knows? What we do know is it is a heck of a lot stronger than it has ever been. Acting like it will continue to run this hot in perpetuity is, maybe not foolish, but certainly not the most likely scenario. (It would be like borrowing to finance a business and stress testing based of today's 2-3% interest rates given the long term average — in NZ — is 6.5%).
Trying to time when the market peaks to sell is a fool's errand. You will only know it has peaked after it has done so. Some people will get it exactly right and that will be much more good luck than good management. All you can really do is look to sell in a rising market (which we have). If you have bought when the market was weaker then you are winning. Don't wait for a falling market because then every other failing investor will be trying to get out what they can as well.
Will the market fall? Again, who knows? It has obviously surged on the back of unprecedented demand created by a global pandemic and never before seen investor interest. Will either of those things continue indefinitely in to the future? The safest bets point to no. Hence, a fall in the market. There is no guarantee of that fall but I would say it is more likely than not. (My business — completely non card/sport related — has experienced a similar surge in demand. This is great, but, I am also not blithely wandering along expecting the exact same scenario to play out in the future.)
The other question to ask is if there is a fall in the market which cards are most likely to be affected? Those are the ones you should look to move (first).
I was going to write a whole lot more but it seems long winded. The short version, I would be more comfortable that guys with established legacies (LeBron, Kobe) and even more importantly not having thousands of new cards produced each year (Jordan) may maintain current values than the current crop of newcomers whose careers have not yet reached the level their prices justify. These guys may justify that price but do you need to wait for them to do it if you can get the return s on your "investment" now.
Now, really to me this is about collecting though. I like my cards — even my non-PC ones! So even if you have a valuable card you don't have to just sell it because it may be less valuable in the future.
" but figured I might try and elaborate my thoughts! 
