oneliner

sed '/./!be;s/"/""/g;/[, \t"]/s/.*/"&"/;H;$!d;:e;x;s/\n//;s/\n/,/g'

script.sed

/./! b exchange

# escape double quotes and fields
s/"/""/g
/[, \t"]/s/.*/"&"/

# append to Hold buffer and restart cycle if it's not the last line
H
$!d

:exchange
x
s/\n//
s/\n/,/g

test

#!/bin/sh

set -ux

unset script
trap 'rm -f "$script"' EXIT
script=$(mktemp)

oneliner() {
  sed '/./!be;s/"/""/g;/[, \t"]/s/.*/"&"/;H;$!d;:e;x;s/\n//;s/\n/,/g'
}

script() {
  sed -f "$script"
}

cat <<'eof' > "$script"
/./! b exchange

# escape double quotes and fields
s/"/""/g
/[, \t"]/s/.*/"&"/

# append to Hold buffer and restart cycle if it's not the last line
H
$!d

:exchange
x
s/\n//
s/\n/,/g
eof


for command in oneliner script; do
  o=$("$command" <<eof
a b c
c d e

123
eof
)

  [ "$o" = '"a b c","c d e"
123' ]; echo $?

  o=$("$command" <<eof
1 2 3
eof
)

  [ "$o" = '"1 2 3"' ]; echo $?

  o=$("$command" <<eof
1, 2 3
eof
)

  [ "$o" = '"1, 2 3"' ]; echo $?
done