Used games are sold in retailers and help subsidise the stores that sell legitimate games and help provide employment for a lot of people in the legitimate economy. Piracy does not.
Plus as you say, 1 copy resold is just 1 copy being resold. 1 copy thats pirated can be turned into millions of copies as is essentially illegal distribution of the software.
Also one is legal the other is not.
Not comparable at all.







