Q: Will deleting listings in Seller Central affect COGS in Accelerlist?
A: All reports come from your Amazon Seller Central so if you listed an item and then sold it, those financials would be captured there and eventually brought into AccelerList's accounting features as you import the reports.
Now, the only thing Seller Central doesn't know is your buy costs and is super important you record those each and every batch. Deleting a listing on Seller Central won't affect accounting in any way.