Formhals, a former Hampshire antiques dealer, is awaiting sentence after being found guilty of fraud at Southampton Crown Court on October 24.
The 66-year-old, who was too ill to appear in court, was convicted on a total of ten out of 15 charges linked to fraud and the forging of signatures including those of Winston Churchill, J.R.R. Tolkien, Elizabeth I, Oliver Cromwell, T.S. Eliot, Marie Antoinette and Pablo Picasso.
The transactions involved tens of thousands of pounds at least.
ABA member Peter Harrington, who worked with the police in identifying the forgeries and bringing Formhals to trial and was an expert witness along with manuscript expert Felix Pryor, is planning an online guide to Formhals' forged signatures in a bid to trace all the items he sold and prevent further problems.
ABA president Laurence Worms, who bemoaned the fact that the books had been permanently defaced, said that collectors must be "bitterly disappointed".
"Formhals' activities finally came to light when his books were offered in the mainstream book trade," he added. "I can only urge book-buyers to think very, very, carefully before buying expensive books from people who do not belong to a legitimate trade association, such as the ABA. Our members are vetted, our books guaranteed, and we have a strict code of practice. Our priority now, as responsible booksellers, must be to isolate and recover any forgeries that are still out there."
At his trial, Formhals had tried to argue that he had offered the books on eBay, along with other items, as 'signed' without any guarantees as to who had actually signed them.
Two of the charges on which he was found guilty related to the possession of articles for use in fraud, but he was cleared of two counts of fraud, with the jury being unable to reach a verdict on three further counts, which were ordered to lie on the file.
Sentencing will take place on December 21 and Formhals has been told to expect a custodial sentence. His assets have been frozen and action will be taken to recover the proceeds of his crimes.