Simple Method to not Post Below Vendor Price
Add a simple check box under auctioning operations to "Never post below vendor price". The addon could check against 'vendorsell' before posting and give a reason for not posting in the post scan results area, something like "not posting below vendor price.".
In the same vein you could put a field for "Never post below x% of crafted price."
If there is no data returned for 'vendorsell' or 'crafted' then the addon could just ignore the request/option/checkmark, as this would be applied after the price was calculated and just prior to posting.
If it was toggled off by default it would be backwards compatible with existing pricing operations.
Just think how much simpler user Auction strings could be. It would make it really a lot easier to get started with TSM.