The part of the auctioneer was a rare on-screen role for former radio actor and frequent voice artist Alan Reed, best known as the voice of Fred Flintstone.
The woman who was a competing bidder for "the thing" was Amzie Strickland, who happened to appear in the previous episode as the nurse who was discharging Laura from the hospital after having Richie. While it wasn't unusual in classic sitcoms for there to be recurring bit players, it's unusual for them to appear in back-to-back episodes.
After the picture is discovered not to be a Wood but a Good painting, Buddy makes a joke to Sally about taking a trip to Hoboken in a cloth coat. Hoboken is where Sinatra (Artanis) was born.
If you look behind the 4 when they realize that they are mistakenly bidding, Rob shouts to try to get the auctioneers attention. The black woman behind them cracks up and ducks her head to try to hide it.
The role of Mr. Holdecker was performed by Howard Morris, who is perhaps best known for his performance of Ernest T. Bass from "The Andy Griffith Show". Morris also directed five episodes of "The Dick Van Dyke Show", from 1963-1965.