Hey Will.
I’ve had relatively good luck combing through flea markets; but the biggest thing to remember is that the things you’ll find there are largely representational of the local scene.
You should also brush up on identifying bekkou and ivory if that’s your goal; as well as their general values.
In Kyoto I encountered folks at flea markets selling bachi for tremendously discounted prices, but the quality and materials were heavily variable. Most were also built with kyoujamisen in mind, rather than more general use.
Min’yo being an extremely broad subset of music means that you should really find what works best for your natural grip and desired sound - unless you’ve got a strict teacher. Then lol do your best.
edit:
In short:
1 - Check Material and Condition, check price. Sometimes you can haggle.
2 - If there’s damage, where is it? The corners of the bachi are extremely important, if there’s damage to both corners you might want to reconsider purchasing.
3 - Keep in mind the requirements of the music you like (and your teacher)