What is a “Billiard Marker?” A quick look at the Ranks, Professions, Occupations and Trades page at http://homepage.ntlworld.com/hitch/gendocs/trades.html tells you that it is “A person who attends on players at billiards and records the progress of the game.”
The same page has hundreds of obsolete job titles that may be found in old documents and even census records. If you have an occupation for an ancestor that you cannot understand, take a look at the Ranks, Professions, Occupations and Trades page on the GenDocs web site. The occupations seem to be mostly for England and Wales but many of them apply to other countries as well.
The GenDocs home page, crated by John Hancock (no, not THAT one… this John Hancock is in England) has lots of other information as well even though it doesn’t appear to have been updated in a long time. You can check it out at http://homepage.ntlworld.com/hitch/gendocs.
Of course, another method is to simply search for the occupation on Google. That’s how I found the Ranks, Professions, Occupations and Trades page on GenDocs.