The first thing in fetching products is to identify the unique identification field. Obviously, ItemID cannot be used as it always change when the product is re-listed. Name is also not a good option as the keywords needs to be modified to match the search results.
So, the best option is to go for Custom Label or SKU(Stock Keeping Unit). You get access to Custom Label field if you are a pro-seller. Or else, some sellers use the SKU number in description field with a pre-defined syntax and that pattern can be used to identify the product.
Once, the unique identification of a product is decided next is to choose which Ebay API to go for. The best option is to go for GetSellerList API. You can use StartTimeFrom and StartTimeTo filter. The range should be less than 120 days. So, you need to figure out the date from which you started listing products on Ebay. Henceforth, multiple calls can be made with date range of maximum 119 days to fetch all the products.
One important thing is pagination. You need to make sure that you have fetched all the pages returned by Ebay. If HasMoreItems is true, then there are more items to be returned requiring additional call.