Agree 100% on 18650. Mainstream 2nd hand cells have specifications and there are pretty good libraries (and old sales listings) where you can find these specs - which will include capacity and discharge amps - regular and max.Real question here; if you find a cell, even if its 10 years old, but you do a capacity cycle test on it, and it tests within 90% of its original published spec, and you build your pack to within those acceptable tolerances... how is this bad? Yes, there might be calendar aging issues later on, but these cells can still provide years of service. At least that what I've always thought...
For example - here's a 'hi amp' 2nd hand cell https://www.batteryclearinghouse.com/products/30-cells-high-grade-lg-mh1-10a-3ah-cells-in-10s3p-pack Notice the "Continous Discharge Rating 10a".
Here's a laptop cell - https://www.batteryclearinghouse.com/products/30-lgabb41865-2-6ah-18650-cells-in-10-grey-modem-packs
Next you have to design your battery. eBike packs typically use 'hi drain' cells (10a or 20a discharge specs) because the battery builder want's a small battery. Laptop cells (the most common type of second hand cells) typically have 0.5a standard discharge and 1 to 4a max discharge - so you can use them but you need 10-20x the number in parallel than a 10a discharge cells for the same current level. However, a 10-20x battery will last a lot longer so it's perfectly OK if you have room for a 10-20x larger battery.
Let's say you need 100a @ 48v battery. For 18650 that would be 14s and each set of parallel cells in the 14s needs to deliver 100a. If you have a 14s20p then 100a / 20p = 5a/cell. If you do 14s50p then 100a / 50p = 2a/cell. And so on. You just have to figure out the max load the voltage and max number of cells you have room for - e.g. the XsYp and then acquire the appropriate cells.
Test procedures will tell you if the cell is OK - regardless of 1yr or 5yr or 10yr or 12yr old history. Testing is straight-forward - charge, discharge at the battery design amp level you want (weeding out heaters). If near original capacity (85% or 90% or 95%) and IR is OK and no self-discharging after a few weeks - then you have good cells.
Note: If you overload a battery of 18650 cells - e.g. pull more current than the cell is designed for, you'll get extreme voltage drops. The more you overload the more you'll see. For example - pulling 5a/cell from 0.5a 'standard discharge' cell will lead you to see a full charge of 4.15v/cell drop to 3.5v instantly 'kind of thing' and then down to 3.3, 3.0 / BMS cut-off within 2 minutes And the cells won't last long in terms of cycles.
This post is just a long winded way of agreeing with @madmax Even new cells become 'old' in a year or 2.
P.S. I use this date code tool - it doesn't always work but more often than not for mainstream cells - https://batterybro.com/pages/18650-date-code-lookup-tool