Ricoh Aficio CL3500N is now printing all documents with lines/streaks all down the left margin of the page. What do I clean or replace to get rid of these unsightly lines printing on every page?

You can try printing a page in black only, then one page each in the 3 other SINGLE colors: Cyan, Magenta, and Yellow. this will eliminate/ confirm one of these causing problem.

the replacement of Transfer Belt Cleaner or Cleaner Assembly is another to try after the above separate-color thing. see if there's a status page of life remaining on replaceable items, including the Transfer belt assy.

i'll assume you didn't scan the job- - this isn't a multifunction scanner/printer type machine?

ok, next in line is to check the Fuser unit, the part on the upper left side/top of printer. if the same streaks appear on the Main fuser rollers, these fuser units cost about 350 bucks. there is also a Fuser Kit, which costs much less, but its job is to add Fuser Oil to the fuser unit and keep paper and ink from sticking.
a guy could suffer sticker shock from the costs of running one of these, considering you could drop a grand buying all 4 drum units, a fuser unit, and a transfer belt assy. . . .