One seat left on this price
 

When checking the airfare on some websites, I often see the message "xx seats left on this price". Are these message true statements or just sales pitch?

Would/could be be true at the time.
But air fares / fare booking class buckets are not fixed. Airline revenue management will change at any time.

Seems like marketing pitch to me.

Marketing stunt - to make take a quick decision.

It means that there is only 1 inventory in the booking class that that fare books into. This can easily be checked on a site like expertflyer. Once that inventory is used, subsequent tickets will result in a higher booking class, which will have a higher fare.

Of course inventory can also increase, for example if someone cancels or changes a flight, or if the airline decides to release more. But this is relatively rare (relative to people booking flights, causing inventory to decrease). So inventory does tend to decrease (rather than increase) over time.

It's definitely not just marketing.

Simple test, do a search for 2 passengers. If the price is more than double that for one person, then it's true. (At the moment, anyway).

In fact if there is only 1 seat left at a certain price (which really just means, only 1 inventory in that fare class) and you search for 2 passengers, you won't get lower price + higher price. You will get 2x higher price, because they will book both in the same fare class. The cheaper fare may still be available after you book the two expensive ones.

On delta, it's definitely a marketing ploy. I've had this happen more than once and after booking nothing changes.

I don't know why people keep saying this. It is most certainly not a marketing stunt. It just means one inventory at the currently cheapest fare class, that's all. This is easily verified.

Well, sometimes/often it is misleading as there will be more seats available at the same price despite the number of seats available in the "warning".

What exactly do you mean? That there is only one inventory at a certain fare class (say, "G") and after you book it, there is later more G space released?

It's certainly possible but it's not the norm, unless you are booking very far in advance.

Yes, I have experienced that many times. Either immediately after making a reservation further seats are available - despite the warning - or the next day.

Its usually a marketing stunt to make you book right away