Thought I would check back in on this. Some good points are made, which I somehow looked past. Thank you guest. All valid points which I will discuss and hopefully it will help this mod.
I got idea about this taxation. Instead of fixed residence tax, why don't you go for income tax instead ? This will work for each time the player sells something to a merchant of some sort. A certain percentage of what money you gain after transaction can be deducted (automatically) by the merchant because he's a good citizen and tax-payer (even if you don't).
If you play a thief type tax-evader, you can negotiate beforehand with said merchant to evade (or reduce) this tax. You can make this with script attached to merchant in towns.
I do not support the tax being auto deducted when you buy and sell from a merchant. It's not their duty to collect taxes, as is apparent by the story line of the dead tax collector and the few others found in other areas. It doesn't make much sense from a role play point of view either. There are many merchants in the game, and vary greatly, many of them are not even honest folk. I know this is taken care of by using a persuasion line like you suggested, but the way you present your idea would mean applying a script that affects every major buyer/seller in the game and adds dialogue to them. This could lead to conflicts, whereas new npcs that do the job are not only more lore friendly, but less likely to cause conflicts.
Perhaps there is a way to sort of keep a tally, but for taxes. If that could be done, you could still use these tax collectors as they would know what to charge. But even then, doing it by how much you buy or sell could mean applying scripts to the merchants. This is why a check of player's inventory was being discussed. It alone would have flaws, but would at least give an idea of how rich the player really is, no matter if they drop gear or not, and when combined with a living area gives it a role play value. It certainly needs refinement, so I am more than open to ideas, but doing it via the merchants alone might not work.
The player also won't be bothered with such scheduled taxation routine (which might be good if you're a craftsman in Balmora but not so good if you're a ranger in Molag Amur with no fixed income). This also avoid player to voraciously piling up money as soon as possible just to overcome the tax.
The ranger out in the middle of nowhere could be mistaken for dead very easily, as he lives "off the land." He'd be an unknown, bouncing from place to place and not staying in one place for long. He'd likely be living in an abandoned house or cave, and too few people would see him enough. His income would not matter, as he wouldn't be coming into town enough to be taxed. In theory few would even know how to find him.
This too needs some work, but from what was mentioned before, it was thought that if you stayed out of towns long enough, it would be thought you were dead. If this was expanded on, like adding a way to forge fake papers after a certain length of time so that your fines go away, a ranger would be feasible once again. Maybe you'd have to go on a repeatable quest for a corrupt taxman that kicks in on a timer of "last seen" or something.
If given a fixed amount by definite period, you won't end up with any good conclusion about the amount to pay. If you're living off the land, you can be level 50 with just racks in your shoulder, yet if you continually buy ilnea's breath scrolls from Arille and sleep just over Seyda Neen waiting for some Dark Brotherhood sap to appear, you can end up with thousands before you're even level 2. Thus 1,000 gold or 5,000 gold won't be relevant anymore.
This was a problem that was being worked on and why a script to check the player's inventory was being looked into. It was thought it would go active each time someone entered a new region or went through a door.. so say someone
did have db armor, the amount due would reflect this. Sure they could drop the gear in a house for display, but to get it there, they have to walk through a door.. so the db armor tax price would have to be paid. But this would not get in the way of displaying your gear in homes, because as long as it was left there in the house for awhile, it would not count against you in the next tax session.
What the concern was is finding the right script to do this, and how to achieve an accurate "average" for taxation, while a person was dropping gear. If done wrong, a person could in theory, carry the gear for only a short way, drop it in a house, and then use it to pay their taxes in full another time when they should have been taxed a higher percentage, or be double taxed for an item. (because they moved it too often in a session or w/e)
If a script could be located and adjusted correctly, much of this would be solved.