Can I be banned from reentering the US because I didn't use my return ticket, but bought a one-way for a later date instead?

The US CBP (Customs and Border Protection) is uninterested in how you purchase your flights from the airline. Their only concern is that you leave the US on or before the date your stay expires. There should be absolutely no problem with your plan.

The only problem that could arise is if your approval to stay is for some reason much shorter than the normal 90 day maximum permitted with an ESTA. This would be unusual, though.

I actually did exactly what you described twice (didn't return on my original ticket) and have entered and left the US without any issue subsequently. No one questioned me about this when I left and it wasn't raised when I entered. The stamps said I had leave to remain for 90 days and on both occasions I left within this period.

I only add this answer because I've actually done what the OP is asking, not because the other answers differ.

Assuming that the stamp you got when you entered allows you to stay until November 9 (which it ought to, barring special circumstances): When you board your one-way flight home, the airline will transmit your passport number to the US authorities, who will then know you have left the country.

It doesn't matter to them that you had a ticket for a different flight that you didn't board -- only that you actually did board some outbound flight before the time you were admitted for ran out.

After you get home, you can verify on the I-94 website that your departure was recorded correctly. In the unlikely case that it wasn't, contact the US consulate for guidance of how to get the fact that you indeed left the US recorded properly in the system.

ESTA doesn't figure in any of this. The terms of your stay are determined at the border when you enter the U.S. There should be a stamp in your passport with an allowed duration of stay. Check if duration of stay is available electronically on the I-94 website as well. For all the U.S. cares for, you could exit to Canada, get arctic paperwork there and walk back to Europe on polar ice later this year :)

Leaving the US on a one-way ticket should be fine. And, as per the other reply, since you will also have a stamp on your passport, you should be good.

On a different note, I personally came across an issue when I went from the US to France and came back on a one-way ticket from France. Basically, I did not use my return ticket to come back from France due to a similar personal situation as yours; instead I came back through Iceland. The authorities in Iceland pulled me and my family (my wife and three-year old daughter) aside and did some additional questioning and enquiries with other authorities. Basically, they were concerned that I did not have a record (in the same airline and transit route) of travelling from the USA to France. I was surprised that I had to spend almost an additional 45 minutes for this.

I’m not sure whether you will have a similar experience, though I guess that as long as you have proof that you travelled from point A to B and back to A, you should be good.