I thought I'd point out a few observations and tips for people in-game about how bazaar taxes work and how buyers and sellers can work together to maximise gil efficiency per transaction.
Clearly, there is a bazaar tax of 10% based on sale price. E.g. I sell ridiculous overpriced gear (Bronze Haubergeon for say 100k, seriously "underpriced" as I understand the Wutai economy) in my bazaar. Upon sale, I suffer tax of 10% or 10k, leaving me with 90k after tax.
Contrast this with a direct trade: I sell at 100k, I pocket the 10% or 10k tax savings.
Bottom line: feel free to negotiate a direct trade of anywhere between 90%-100% of the bazaar price (although your discount on purchase diminishes the closer you get to the bazaar price).
In fact, the vendor is likely to want to sell you the product slightly cheaper because a) their after-tax return is greater than if they did a bazaar sale, and b) they are interested in clearing a bazaar slot, and turnover for vendors is important when maximising profit.
On direct trade, your maximum discount is an 11% decrease in the cost of an item per direct trade (assuming you transact at a price equal to 90% of the bazaar price).
You could also try negotiating the price using raw materials relevant to that crafter's discipline to lower the gil price (you offload useless items from inventory).
Bottom line: you can increase your after-tax profit by accepting a lower direct trade price anywhere between 90%-100% of the bazaar price. You can also build goodwill with customers who know what you sell and understand that you are open to negotiations.
On direct trade, your maximum upside is an 11% increase in after-tax profitability per direct trade (assuming you transact at a price equal to 90% of your bazaar price).
Effectively, apart from the time to take an item down in the bazaar and execute the trade, the vendor is no worse off financially for trading directly with the customer.
Alternatively, if you are offered ingredients, having an understanding of their cost will help you work out if you can use the materials to generate surplus ingredients for future profitable syntheses.
Everybody is better off by trading directly. People rarely bite when it comes to trying to sell a product, so feel free to negotiate with them, because by doing so, you're actually increasing the benefits to both yourself and the counterparty.
Edits for clarity
Some posters have mentioned that retainer bazaar sales attract a 5% tax. Notwithstanding, you can't negotiate with an NPC or leave it messages, therefore you are stuck paying the vendor's price and hence a tax inefficiency arises. Therefore, my conclusion still stands that the most efficient means of sale is direct trade (unless the 5% also applies to actual characters standing in market wards, in which case they're probably AFK anyway).
Edited, Oct 6th 2010 1:09pm by Dik
Edited, Oct 6th 2010 1:11pm by Dik
Edited, Oct 6th 2010 1:13pm by Dik