If all your listing contain a the same word, you could use the “Bulk Search & Replace” command to replace these words with something different.
You could use a short AppleScript like the one below to set the description of each of the selected listings to something else.
tell application "GarageSale"
repeat with theListing in (get selected ebay listings)
set the description of theListing to "My custom description"
end repeat
end tell
In both case you’d need to use the “Revise Listing” command to upload your changes to eBay.