I agree.
When I made trades with my steward I finished the deal with my own bonds. In true I needed create bonds to make the deal and then I will need to request compensation from my liege.
If the whole system was based in tax, then the steward, or lord, will not need to have the gold, or bonds, to make the deal.
Only traders would have to have bonds to broker a trade.
i don't think steward paying out of own pocket is that big an issue. i mean.. stewards could also be making a bit on the side if they are doing the selling. they should ask for a bigger estate (higher income) to cover trades if they are in buying regions. only thing is possibly a steward cut or something off the lord's cut if their estate is maxed and still not enough (unlikely)
and.. no.. traders don't need bonds, since they are not allowed to make a loss anyway. so they can't use up bonds when brokering.
the problem with going tax was that you could end up with negative income for the region.. for poor regions.... and kill off a load of buildings, etc... in return for "free" food. (and no income)
so i think if you are going back to the tax route, some sort of region coffers that gets filled from the lord's tax and / or manually paid in by the lord would be needed. no funds in the coffer, no trade.
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it might also be useful to allow a lord/steward to put up offers anywhere. but obviously still cannot fulfil an offer unless at a market.