Impossible to answer without a detailed description
Marksman made several models. Which one are you asking about.,
There have been several models of the marksman repeater. The first one was made in 1955
I have one
If you want quality then there are no cheap BB guns, Their are just cheap BB guns that when they break, they are not worth fixing. They are poor in power and not very accurate. The cheapest line of BB gun are made by Marksman. An example is the Marksman model 1010. It cost under $30 and has less power than a Daisy Red Ryder. The Marksman line of air rifles are also cheap and not accurate. The next one up from the Marksman is the Daisy Red Ryder. They are mass produces and don't have serial numbers, they have BATCH numbers, because they are made in the thousands. Surprisingly one of the nicest Pellet guns that is customizable only cost $58 and it's a .22 caliber CO2 pistol. It's the Crosman model 2240.
You probably can't. Marksman doesn't list all of the 45 plus BB guns that they have made over the years on their website. You most likely have an older one not listed. You need it go to a book store and find a "Bluebook" on airguns.
100 USD or so
The sn should start with 69m. That would be one made in 1969. Without more of a description the best value range I can give is 500-1000
Selling Belgium in deactivated condition for 325 euros (£270 +) and simillar prices in UK if you can get one
i bought one last year for $100, it was a rhom, in 22 caliber
I just picked one up for $80
They only made this boys rifle for 5 years and they are difficult to find in good working condition. I have one in my personal collection and value it at $200-$250
Not if you won't tell us the make and model. More than one company made firearms in 38-55 caliber.