Not a bad idea to have them be able to expend trade points during downtime to earn some money. Though I might make it as simple as trade points spend = gp earned, possibly with the addition that you can support a certain lifestyle (so the gp earned is all profit), or that each lifestyle can be supported by spending a certain number of tradepoints (less than the gp that would be required), and anything beyond that is profit.
I'm also playing with an idea for an alternate subclass capstone. Basically, what I was thinking was lifting the limit on how many item you can make, but I still want to limit the value of items you can make. I think the best way to do this would be something like regaining 1 trade point at the start of each turn. That way, you can still spam vials of acid all day, but you can't make anything worth more than 500 gp (at 20). Though this would mean unlimited healing, since you can make healing potions. You can also make spell scrolls, and can use them yourself since you get Use Magic Device. So maybe this isn't such a good idea.
Hmm, maybe a better idea for an alternate capstone would be to just reduce the trade point cost to create something by 1. This means anything 25 gp or under doesn't cost any trade points to create, letting you make unlimited of them (healing potions are 50 gp, so they'd still cost 1 trade point to create). This would greatly extend your use of low value items (and slightly extend the use of high value items), and insure you can always chuck a vial of acid as a bonus action, even when you're out of trade points. But would this be better than their current capstone? What if, since they seem to be a bit on the underpowered side, we just gave them this in addition to their current capstone?