Check the date code on the barrel, the details and table lookup are on the Remington Society of America, manufactured date link.
50-200 usd
what is the market value of a remington 20 gauge semi-automatic sportsman 58 shotgun?
Your question answers itself.
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Remington
75-400 USD
First of all you have the "sportsman 48", "sportsman 58" and the "sportsman". If it is a plain sportsman it's value depending on it's condition is from 150 to 200 dollars.
Take it to a professional
Take it to a gunsmith
In good condition about $350
Go to http://stevespages.com/page7b.htm
About 275 in good condition