Remove space above `itemize` list within table environment

• If you use the compactitem environment, you need to encase it in \begin{minipage}[t]{\hsize} and \end{minipage} statements.

• If you use an itemize environment along with the enumitem package, you can add the statements

  before={\begin{minipage}[t]{\hsize}},
  after ={\end{minipage}}
  

  to the list of options that go with \begin{itemize}.

\documentclass[12pt]{scrbook}
\usepackage{paralist,enumitem,tabularx}
\begin{document}

\begin{table}[ht!]
    \setlength\extrarowheight{2pt} % optional, for a slighly more open "look"
    \setlength{\leftmargini}{7pt}
    \begin{tabularx}{\linewidth}{l | X | X }
    \hline \hline
       & ECB & FED\\ 
    \hline
    M1 & \begin{itemize}[nosep, left=0pt,
                         before={\begin{minipage}[t]{\hsize}},
                         after ={\end{minipage}} ]   
         \item point1  
         \item point2 
         \end{itemize} 
       & \begin{minipage}[t]{\hsize}
         \begin{compactitem}  
         \item point1  
         \item point2   
         \end{compactitem}
         \end{minipage}\\
    \hline
    \end{tabularx}
\end{table}

\end{document}

For the space above, it is enough to make LaTeX believe the cell is a minipage. For this, I borrowed a \compress command found in an answer to a question on this site. For the space below, you have to add a negative \baselineskip at the end.

\documentclass[12pt]{scrbook}
%\usepackage[autooneside=false, headsepline, plainheadsepline, automark]{scrlayer-scrpage}
\linespread{1}
\usepackage{paralist}
\usepackage{enumitem} % enumerate?
\usepackage{tabularx}
\makeatletter
\newcommand{\compress}{\@minipagetrue}
\makeatother

\begin{document}
some text
\begin{compactitem} \item point1 \item point2 \end{compactitem}
\begin{table}[ht!]
    \setlength{\leftmargini}{7pt}
    \begin{tabularx}{\linewidth}{l | >{\compress}X |>{\compress} X }\hline \hline
        & ECB& FED\\ \hline
    M1 & \begin{itemize}[nosep, after=\vspace{-\baselineskip}] \item point1 \item point2 \end{itemize}\\
\hline
    & & \begin{compactitem} \item point1 \item point2 \end{compactitem}\leavevmode\vspace*{-\baselineskip}
       \\
\hline
    \end{tabularx}
\end{table}

\end{document}