Your mistaken about the differences between the 2 model receivers & the value.Something to remember about used Mac gear is that each model has it's own seperate resale value,things like how many units were made & rarity affect pricing drastically.
Your Mac 4100 was made for allmost 10 years & massive amounts of that model are on the market, the Mac 4200 was only made for 3 years with very limited numbers of units sold & on the used market.
Going from memory on this so i may be off on a component but these are the internal components of the 2 different models.
Mac 4100 resale value = $400 to $600
Mac 4200 resale value = $800 to $1,000.
Mac 4100 = 75 wpc.
Mac 4200 = 100 wpc.
Mac 4100 tuner section = MR-75.
Mac 4200 tuner section = MR-7082.
Mac 4100 preamp section = C-29.
Mac 4200 preamp section = C-30.
Mac 4100 amplifier section = Mc 752.
Mac 4200 amplifier section = Mc 754.
As predicted the unit was not for sale very long & was snatched up quick,more likely than not it will turn up agin in a month or two with a hefty price increase.