GameStop 's mysterious financing letter underpinning its audacious $56 billion bid for eBay is emerging as a central issue in the proposed takeover, as questions mount over whether the deal is actually financeable. The video game retailer said it has lined up a $20 billion financing commitment from TD Securities, part of TD Bank. But a key condition attached to this letter could ultimately make or break the deal: the combined company would need to maintain an investment-grade credit profile, CNBC's David Faber reported, citing people who have seen the document. Moody's Ratings said Wednesday the proposed acquisition would be "credit negative" for eBay because of the substantial increase in leverage implied by the deal structure. The ratings agency estimated leverage for the combined company could approach nine times debt to earnings before interest, taxes, depreciation and amortization before accounting for any cost-saving synergies.…