Hi Caleeb. First... since I see your button-ID request is your very first post here at TreasureNet, let me say, "Welcome to T-Net's What Is It? forum, the very best lace on the internet to get unknown objects CORRECTLY identified." (BTW, never trust what an Ebay seller says -- and unfortunately too often, what you read at WorthPoint.)
You're correct, the typical relic-hunter's name for your find is a flatbutton. More specifically, collectors of Historical buttons call it a "1-piece" brass flatbutton. (The shank/loop is not counted as a piece.) The first 1-piece brass flatbuttons with a backmark (maker/dealer's name or quality-level marking) were made in Britain. Your backmark saying "Superfine" is a quality-level, which was originally set up by the Button-Making Guilds in Britain. For example:
Fine Quality
Extra Quality
Superior Quality
Superfine
Double Gilt
Treble Gilt
The word "Plated" in a backmark always meant Silverplate -- Goldplate was Gilt
The first backmarked flatbuttons had the backmark written in raised lettering. They start showing up in Britain "about" 1790, perhaps a few years earlier. A backmark done in indented lettering seems to have begun about 1810 -- perhaps a few years earlier. So, your indented backmark gives us a starting point in time-dating your find. The ending-point would be about 1835, when plain-front flatbuttons fell out of favor with the public due to the advent of machinery which could mass-produce INEXPENSIVE hollow 2-piece brass buttons with an "ornate" design or emblem on the button's front.
Because you dug your flatbutton with a very simple one-word indented backmark in the US, my guess for its date-of-manufacture is the 1820s. That is when the infant American button-making industry first became capable of manufacturing enough metal buttons to meet the demand from the garment-making industry. I should mention, a factor in my guess/estimate is the fact that the supply of imported buttons from Britain got cut off by the War-of-1812... and afterward many patriotic Americans boycotted British-made goods for about 10 years after the war ended in 1815.