Revenue (net sales)
This helps to determine a supplier’s overall financial health and bargaining power. If they are performing well in the market, you can put in a stronger bid.
Gross margin (%)
By calculating the difference between a supplier's revenue and the cost of goods sold, you can understand how profitable the supplier is and negotiate a better deal in case of higher margin availability.
Debt to equity ratio (solvency)
If you know the extent to which your supplier is using debt to finance their operations, you can use that information to gain financial leverage over the negotiation process as per the supplier’s financial stability.
Operating profit margin (%EBIT)
By calculating the difference between a supplier's operating income and revenue, you can gauge the supplier's profitability. If high, you can bargain more and close yourself a better deal.
Liquidity (current or quick ratio)
Understanding a supplier's financial stability in terms of their liquidity position can also help you understand to what extent you need to bargain.