Is there a way to change the maximum transaction amount of outpost vending machines? e.g. in the shop that sells wood for stone the maximum transaction amount is set to 30 - I know it's possible to change the prices but is there a way to increase this maximum of 30 without changing the prices itself?

The max transaction count is determined by the amount stocked in the vending machine (the amount you see in the map marker). It looks like the plugin has features to configure that, but a recent post seems to indicate that the feature no longer works.

https://umod.org/community/compound-options/41687-question-regarding-refillsout-of-stock

For the purpose of editing monument vending machines, I recommend using the Custom Vending Setup plugin which improves the user experience of editing vending machines, provides deeper configuration options, and integrates well with other plugins such as Monument Addons and Talking NPC Vendors.

Thanks a lot for your response, I was indeed able to edit it using the plugin that you mentioned.