I haven't. I've tried valued stones, magatama's, weapons, etc... nothing is selling. All minimum.
In popular spots such as byak, forest, etc...

Any advice?

~wishes there wasn't a min~

It really depends on what you're selling, generally there's specific maga that'll get sold with no doubt usually being the ones with skill force. (at least that's what I've been able to sell)
It's kinda heavily reliant on knowing what people want/seeing what other people are able to sell so I'd guess just spy on other people's stall to see what they sell and try to emulate it lol. Putting your stall in a good area range to when people will need them helps for selling as well.

Its pretty much all about putting affinity in low level areas, and skill force in high level ones imo

I've noticed that Mushiro magatamas are pretty popular in Kumaso.

to be honest I've only sold like maybe 4 items by luck lul.

I've made about 120+ C from sales I think.