Start with small companies when recruiting B2B clients. They are more open and willing to collaborate, have fewer decision makers, and the entire sales process is shorter. Then go to medium companies, and finally to big companies. By the time you reach big companies, you'll have enough supply/demand in your platform to be attractive to them.
Generate demand for your supply as soon as possible, even in small quantity. Otherwise, suppliers lose trust in your platform.
Balance supply and demand to prevent either party from becoming frustrated.