British criminal historian checking in.

No, your ancestor wasn't buying chickens! A "counter" (also known as a "compter") was a kind of debtors prison in England & Wales, although very rare in Wales, that also dealt with summary public disorder offenses before Police Courts were a thing. They were run by the local authorities, which in the 1400's would have been a non-noble sheriff appointed by the elected officials of the counties, and later on, the boroughs.

They were fascinating places. Basically, if you owed money and a successful prosecution was pursued, you were sent to a compter - but they were very different from prisons nowadays. If you were an aristocrat who owed money to the Crown or another nobleman, you would never tolerate slumming it with a commoner - even in prison - so they had separate suites, and even servants to wait on them!

The biggest one in London was the Wood Street Compter. The one you're after - Poultry Compter - was also in London, at Cheapside, which is now ironically the centre of London's financial district.

OK, so the debt. As you're likely aware, a 'mark' has never been a denomination of British currency. It was an abstract amount worth worth about two thirds of a pound (13s 4d in old money), and for some reason it was used widely in the accounting profession. I'm not quite sure why. 100 marks would have been between £66-£70, which was a fair old wad of cash back in those days. Prior to the formation of a national banking system (that came much later in the c. 17th) or any kind of recognisable civil law enforcement authority to enforce payment, commercial and private debt was a hard fact of life. Transactions were based upon trust, not collateral, and business debt usually centred around non-payment for goods and services.

The source is unclear on who owed who though, from what you've said. "Brought actions" meant then what it does now - an individual pursuing a prosecution. This was way before the Crown Prosecution Service. Without any public prosecutor, legal recourse of any kind was only available for those who could afford it. It's quite possible that your ancestor was sued for 100 marks by the Grocer, wrongfully in his eyes, and he spent 5 years attempting to recover the debt from the person who brought charges against him. Legal proceedings were often held at places of imprisonment, especially for minor offences or civil matter, so "at the compter" likely just means that the case was heard at the prison.

In terms of further research, I'm not well-versed in compter records, and summary judicial records in particular are notoriously hit-and-miss if you're looking for court proceedings. If you've got the time to trawl through microfilm of c. 15th century court scrolls then you may find something in a London archive, but it's proper "needle in a haystack stuff". If you're so inclined, the National Archives are probably your best starting point.

Best of luck to you!