It would be great if people could sell their old drop shop purchases to other zwifters, the price would be lower than the new versions depending on how old they were and their performance could be slightly worse as a result.
would be kind of cool, so you could sell your old bikes to get extra points to spend on new ones
Right now that would be against Zwift ToS:
Restrictions. Virtual Items are non-returnable and non-refundable. Virtual Items cannot be resold, transferred for value, redeemed for cash or applied to any other account. We may limit the number of Virtual Items you can earn or use within certain periods of time or implement other restrictions on the receipt or use of Virtual Items. We may further restrict your ability to redeem your Virtual Items based on your place of residence.
But if I could sell anything it would be my unused Tron bike. Any takers?
I think CJ needs another one. 
Or sell them back to Zwift.
A 2nd hand market makes no sense but if you could return a bike you bought and get some of your drops back (maybe 50%) then it might actually make the drops shop worthwhile.
I think the op means an in-game auction house where you trade drops for used equipment. That wouldnāt be a tos issue.
yes - this is what i meant - not for real money
Question on this proposition: Would the used items still be ālevel lockedā like the original? That is, if you have to be Level 20 to buy bike X, and you later wanted to sell it, would the buyer also have to be Level 20?
Also, at least from my perspective, drops accumulate REALLY quickly. So why would someone buy something used, and slightly slower (per the OP), instead of just waiting a little longer and getting a new one?
The issue I see with the present set up is that you run out of things worth purchasing reasonably quickly (presuming the purchase is performance-based, that is) because the testing for what is the ābestā for different terrains/surfaces has been done.
Personally, I am still in favor of the purchased items each having their own āhealthā and needing to spend Drops for maintenance on a regular basis. For example: tires to be replaced, chains to be replaced, and even though kits arenāt through the drop shop (at least not yet), Lord knows we can see through those bibs youāve been wearing for the last 15,000 miles!!! 
Iād be happy if I could just get rid of some of the stuff cluttering up my garage. Maybe we could donate items to those blue ghost riders I havenāt seen for ages!
I would agree to a trade-in of old equipment toward the purchase of new gear with your own drops, but I think you can purchase some really good gear early in the game, so a used bike shop in Zwift might not get the attention of the users as much as sitting in the saddle for one more level and buying new.
The āhaving to perform maintenanceā idea is interesting, though. Letās say I havenāt ridden in a few months, I left my Specialized Tarmac Pro out in the rain by accident, and Iām really low on drops. Now I gotta do a couple KOMs on Alpe du Zwift at half power (with the squeaking chain effect added in-game) in order to replace my rusty chain, true up my wheels, and get new handlebar wrappings.
NOW weāre getting into virtual riding!
XPās (levels), equipment, badges, etc. are ways to create targets to achieve for players. In that sense they are personal progress indicators that can also be shown to others. Being able to exchange them would create a social element to the game, but it would ruin the value of showing āI accomplished thisā to others.
One problem is that they are not really āusedā are they! So how to value and why would anyone buy new? Unless the performance was reduced slightly with each transfer 
The idea of being able to sell back to Zwift, maybeā¦
You do realize that the equipment does not really wear on Zwift, right?
You do realise itās not really equipment on Zwift, right?
Of course, I realize that. But the OP (as noted in my comment) had suggested that āusedā equipment might be assigned a ādecreased performanceā by Zwift.