Your Current Software
You can find an option to download a CSV of your existing listings usually, if you are using a similar software. You'll need to download that file and open it in a new Google Spreadsheet.
There are a few softwares that don't allow this option. You will need to email them to get a copy of yours.
In Google Spreadsheets
To open a New Spreadsheet click the green plus in the top left after you open the link above.
After you are inside the new spreadsheet, click File > Import. Then choose the Upload tab on the new window that opens. Now click Select a file from your computer in the blue box, or you can drag and drop the CSV file if you have the download bar on the bottom of your screen.
You'll probably have several columns. You only need 2 of these columns. You need the column that has your eBay ItemIDs and you need the column that contains your ASINs or SKUs. Delete all other columns.
You should be left with Item IDs in Column A, and ASINs in Column B.
Now delete row 1 to delete the headers. You should only have numbers in column A and skus in column B
Here is an example of what you should see. Column A is the eBay listing ID and Column B is the source market ID or the ASIN.
Now click File > Download As > Comma Separated Values (.csv, current sheet)
You can now go to your Easync account and go to Tools > Sync Unknown Listings page. Be sure you have the correct source market chosen from the drop down list, then click "Choose File" and find the CSV you just downloaded from Google Sheets. DO NOT choose the original CSV that you started with!
You must have the CSV you just downloaded from Google Sheets