How to render a double bottom border on a single table cell?

Typically one could use hhline to modify the horizontal rule within a tabular, allowing a mixture of single and double lines. However, the output in your case is not very desirable. Here's an alternative, showing a modified version of your input (using booktabs) first with \hhline and then without (and some other tricks):

\documentclass{article}
\usepackage{booktabs,hhline}% http://ctan.org/pkg/{booktabs,hhline}
\begin{document}
\begin{tabular}{lrr}
  \toprule
  Item  & Sales & Total \\
  \midrule
  Item 1 & 10 &    \\
  Item 1 & 10 & 20 \\ \hhline{--=}
  Item 1 & 10 &    \\
  Item 2 & 10 &    \\
  Item 2 & 10 & 30 \\ \hhline{--=}
\end{tabular}

\bigskip

\begin{tabular}{lrr}
  \toprule
  Item  & Sales & Total \\
  \midrule
  Item 1 & 10 &    \\
  Item 1 & 10 & 20 \\ \hline \\[\dimexpr-\normalbaselineskip+\arrayrulewidth+1pt]
    \cline{3-3}
  Item 1 & 10 &    \\
  Item 2 & 10 &    \\
  Item 2 & 10 & 30 \\ \hline \\[\dimexpr-\normalbaselineskip+\arrayrulewidth+1pt]
    \cline{3-3}
\end{tabular}
\end{document}

The second option inserts a regular \hline followed by a 1pt gap and \cline{3-3} (modify 1pt for a different gap). More specifically, \dimexpr allows for an expression of dimensions, jumping back (vertically upward) a full baseline skip (-\normalbaselineskip) and then forward (vertically downward) the default width of the rule (\arrayrulewidth, typically 0.4pt) plus 1pt.

I'm sure another technique would also suffice; this just seemed the most straight forward. This is mainly based on style, so there are other options as well.

The later option needs some additional hacking when using vertical borders also,

\documentclass{article}
\begin{document}
\begin{tabular}{|l|r|r|}
  Item  & Sales & Total \\
  Item 1 & 10 &    \\
  Item 1 & 10 & 20 \\ \hline 
  & & \\[\dimexpr-\normalbaselineskip+\arrayrulewidth+1pt] \cline{3-3}
  Item 1 & 10 &    \\
  Item 2 & 10 &    \\
  Item 2 & 10 & 30 \\ \hline 
  \multicolumn{1}{l}{} & \multicolumn{1}{l}{} & \multicolumn{1}{l}{} 
  \\[\dimexpr-\normalbaselineskip+\arrayrulewidth+1pt] \cline{3-3}
\end{tabular}
\end{document}

hhline is designed to make such partial lines easy to produce:

\documentclass{article}

\usepackage{hhline}

\begin{document}

\begin{table}[htbp]
\begin{tabular}{|l|r|r|}
\hline
Item  & Sales & Total \\ \hline
Item 1 & 10 &  \\ \hline
Item 1 & 10 & 20 \\ \hhline{|-|-|=|}
Item 1 & 10 &  \\ \hline
Item 2 & 10 &  \\ \hline
Item 2 & 10 & 30 \\ \hhline{|-|-|=|}
\end{tabular}
\end{table}

\end{document}