@jwildeboer empirically my 5 year old Kia e-Niro shows essentially no visible degradation at all. Last summer I was still getting 280-300 miles range on mixed driving scenarios, which is over the official stated 270 mile range at new 
The only common cars where you had significant degradation were first generation nissan leaf which lacked any battery thermal management and a small battery to begin with. I wouldn't hesitate to buy another secondhand EV - much less to worry about than a secondhand ICE with countless moving parts to go wrong.