10 years ago, when I was in high school, I bought everything used, paying no more than 70% of the new price for a used pedal. But, in the last year, I've bought about 5 used pedals, and 2-3 of them have had issues within months (scratchy pots, scratchy jacks, power supply issues).
Overall though, of the 11 pedals that I own, 5 of them are used. So about 50%.
But I do use a few rules.
1. If I know I'm going to have the pedal for life (Fulldrive 2), I'll generally buy new.
2. If it's a pedal that's had issues in the past (bad switches, pots going out) or is known for wearing out, like a volume pedal, I'll buy new.
3. If it's just a back-up pedal, or one that won't be a regular on my board (extra overdrive), I'll buy used.
4. If it's over $200 new (Fulltone Clyde Wah), I will generally buy used.
5. If it's boutique, chances are there are very few used ones for sale, so I have to buy new.
6. If it's built like a tank (Rat), I'll buy used.
I like playing guitar.