Why will lever action rifle sellers Never state the barrel length?

Probably because most leveraction rifles have very standardized, common barrel lengths. Winchester model 94 30/30 of any ivntage after WW ll? 20 inches, baby. Got a 348 or a 250/3000? Probably a 24 out there. If a non standard barrel lenght like the 20 inch carbine barrel on some Model 71's or a 16 inch trapper barrel or a 26 inch octagon barrel on some 30/30's then yes it should be stated.

"Generally" by the model of a Marlin you will know the barrel length.