A 2/3's mag 336 with 20" barrel would indicate an "SC" model manufactured 1948-1963; a half-magazine model with 24" barrel would indicate an early 336A model manufactured from 1948-1962, and 1972-1978 (if my memory hasn't failed me again). Models in 30/30 Win are not hard to find and don't bring a premium if not in really nice condition; but models in 35 Rem and 32 Special are not as easy to find (esp. in 32 Spec), and will bring a premium over a 30/30. Marlin also made a couple of other models with 2/3 mags and 20" barrels; early Glenfield 30-A models and the 36-G, but these are not marked Marlin although they are identical except for features.
I missed on a 24 inch 336A in 30/30 three years ago and couldn't find a clean one at a decent price for a long time. Finally got one of gunbroker before the holidays for $395..plus $25 shipping and $25 transfer fee. I'd buy this over a 44, but I already have 44 Cowboy rifles...I'd like a second 336A in 24 inch. (The round barrel 44s seem to show up on the used racks fairly frequently...forget about 357s.)
Any clean used MARLIN is going to be pricey. Four out of five of the gun/pawn shops I frequent DO NOT have any used MARLINS in the rack...nothing new either!