My online execution-only broker has contacted me to say I have overbought shares and am now financially liable.

I have been dealing very frequently - sometimes over 60 times a day and have relied on the integrity of the company website for account details.
My online dealing page had previously shown that I had shares to sell in several companies which I later sold.
The broker now says the details shown on the dealing page were wrong - that it was a mistake to say I had these shares in my account
- and that the shares have now been bought back from the market - leaving me with a financial deficit.

Am I liable for this deficit - or does the fault lie with the broker for telling me I had shares to sell which I did not have ??

If i was your broker and this happened there would be one solution.

You would be right every time and problem sorted no matter what the cost.

"60 trades a day"

Most brokers would bite your hand off to have customers like that.