One customer buys every month 1000 units for cash and the other customer buys 1500 units on 30 days credit. which is a good deal for the seller?

It all depends on the financial health and the business growth of the seller.

If the seller is financially healthy and can bear credit sales, the buyer who buys on credit terms suits the seller best as there will very limited sellers to entertain such a buyer.

My Challenge will be to assess the real requirement of the Buyer--If his capabilities incomparable as that of the Cash purchaser definitely I consider the credit party--many credit party enjoy the facilities not because they are not having cash liquidity, but because of credit worthiness.

Cash players are not sticky--when they get a better chance, even they may not allow a negotiation chance. In other sense also, when they lack credit worthiness they are forced to buy from the market on cash basis.