a professional shower curtain ring seller?

It's a movie. A light, comedic movie.

The shower curtain ring seller character is a satire on travelling salesmen and the ridiculous ways they made a living during the 80's. When I was 22 years old I had a salesman come over to my house in 1988 on a weeknight and spend a couple of hours selling me car insurance. He came over in a suit and tie plus the obligatory briefcase. He laid it on thick but I bought the stupid insurance, something like $400 for six months, kinda expensive(I made about $1,200 a month) because I was so young.
I guess what I'm saying is that's the way it was back then, poor schlubs with families(unless they're widowed) were willing to travel across the U.S. and actually sit in peoples homes in an effort to pay the mortgage, all the while sucking up to their corporate masters.
Dell Griffith was one of those travelling salesmen. He flew coach on crowded airplanes which waited in queue every takeoff because there was a shortage of Air Traffic Controllers. He ate plate specials at greasy spoons, smoking his cheap Paul Mauls to deal with the stress of meeting his quota. He kept track of his expenses in a small notebook with gravy stains on the pages. He read countless USA Today's and local newspapers, endlessly filling out crossword puzzles in bus and train stations. He drove his pathetic Mercury Tracer rental from one Motel 6 to another. And yes, he washed his size XXL underwear in sinks if he had more than one night stay, which often he did not. He was required to call corporate every Friday and give them his sales numbers, hoping to stay just ahead of the deadly axe. Holidays off? Forget it.

DEL IS A DOOR TO DOOR SALESMAN...IN THAT PROFESSION..YOU SLING WHATEVER THE COMPANY TELLS YOU TO..EVEN POSSIBLY MAYBE SHOWER CURTAIN RINGS.

No, not in the '80s but back in the day? (whenever that was)

it was a joke and never meant to be taken seriously in terms of plot viability. The point was the point of Dell's character as a man of the road who had to sell all types of trinkets to get by. Remember that film sequence when he was selling anything to raise some money for himself and Neil? He was selling his shower curtains as earrings to willing buyers.