Pros
Getting to learn about and play with lots of different gear and instruments. Sometimes getting a decent deal on some neat gear.
Cons
Besides a slight discount, no other benefits whatsoever-- such as paid time off, paid holidays, health insurance, etc.
Individually owned franchise, so usually little to no upward mobility unless the owner has a manager position.
In my experience, in all the years I worked there I think I received two small raises (besides a mandated raise when the federal minimum wage went up higher than my previous wage...), despite making the store/owner a LOT of money over the years. My experience with most owners has been that they are greedy and care little for how their employees are doing, as they are well aware that there is a huge crowd of burnout hack musicians that would love to work there and they'll take whatever pitiful hourly wage the owner offers.
All Corporate cares about is their cut of what their franchises make. You'll likely have very little interaction with them, which I guess could be a "Pro"-- but you can bet that they won't have your back if needed.
Buying used gear from people often results in a conflict with customers. People treat you like you're on Pawn Stars.