Any way to adjust the maximum number for scrap in vendors? Afaik the maximum for a buy order is 1k, while the sell order is 9999. Possible to increase both? Could it be a plugin?

Had a quick look at this, there is sadly hard-coded clamping involved here. The clamps are:
- currencyAmount between 1 and 10000
- itemToSellAmount between 1 and the max stackable amount for that item
You can override the amounts in the "OnAddVendingOffer" hook, but at that point the amounts have already been clamped so you don't know what the amount is that the player wishes to set.

Sadly it won't be easy to do this elegantly.

Thank you for looking