Is your feature request related to a problem? Please describe.
Sometimes it happens that players put items in the auction house at a loss, just because they want to be cheaper than the lowest seller. So it happens that the items are put in cheaper than they would be worth at the vendor.

Describe the solution you'd like
It would be good if there is a setting that prevents creating an auction with a loss.

Quick and dirty solution
There is a function that returns the button state for creating an auction. https://github.com/Auctionator/Auctionator/blob/master/Source_Mainline/Tabs/Selling/Mixins/SaleItem.lua#L482
Here it would be possible to add another check to prevent selling below the vendor price.

function AuctionatorSaleItemMixin:GetPostButtonState()
  return
    self.itemInfo ~= nil and
    self.itemInfo.count > 0 and

    C_Item.DoesItemExist(self.itemInfo.location) and

    -- Sufficient money to cover deposit
    GetMoney() > self:GetDeposit() and

    -- Valid quantity
    self.Quantity:GetNumber() > 0 and
    self.Quantity:GetNumber() <= self:GetPostLimit() and

    -- Positive price
    self.Price:GetAmount() > 0 and

    -- Price is higher than vendor price + deposit
    self.Price:GetAmount() > (select(11, GetItemInfo(self.itemInfo.itemLink)) + self:GetDeposit()) and

    -- Bid price is not bigger than buyout
    self.BidPrice:GetAmount() <= self.Price:GetAmount() and

    -- Not throttled (to avoid silent post failure)
    Auctionator.AH.IsNotThrottled()
end

Reviewing this request, requirements for the feature:

1. Shouldn't be any confusion about why the posting is blocked for a user
2. The message about the vendor price being the better option should interrupt the posting so it doesn't get ignored.
3. Should be turned on by default so all users are protected.

Adding a tooltip to the post button to inform a user why it is disabled would solve 1 and 2.

Another option would be a dialog popping up to get the user to confirm the post if its below the vendor price + deposit amount.

Added to the latest alpha for retail

Also available in TBC now