This is the reason why we've had to remove vendors in the past. It's too bad that it still occurs with some vendors, was hoping by now that things may have changed for them.
It can be difficult at times being the dealer between a customer and the manufacturer, did that for many years. The manufacturer will tell you an item is in stock, or will be shipped on a certain date, so you tell your customer that. When something happens and the part doesn't ship on time or is lost in transit you get the blame.
On the other hand, taking money for an order and then not ordering that part for weeks or months because you need the money to pay other bills is completely wrong and criminal as far as I'm concerned. Being self-employed for 25 years I see how it happens, but that does not make it right. And it's becoming too prevalent in the car parts industry.
I have a good friend who runs a warehouse for a motorcycle/jet ski/snowmobile parts warehouse. They are very large and very profitable. And they treat their customers like gold. I mentioned to him that his boss (who was looking for other write-offs and investments) should start an automotive side to his business, probably in the pro-touring side which is really growing. I think there's a market for a large vendor with good pricing, excellent customer service, who actually does what he is supposed to all the time.