Here, ebay, ...
Parts that sell:
unique, out-of-production parts that are needed to repair items that people want to preserve (vintage video games, vintage computers, ...).
Expensive parts like high resolution ADC if sold at a 90% discount and shipping costs are not high.
Parts that don't sell:
- Op amps, logic chips, resistors, capacitors, LEDs - especially if the are easily purchased elsewhere.
- Anything priced near the retail price
- parts that are grab-bags of a mix of parts
What are you selling?