As Willy tries to plant those seeds, he is quoted as saying:
Nothing’s planted. I don’t have a thing in the ground
The seeds are a symbol of Willy Loman's failed efforts to achieve the tenets that he lived by, and which he considered essential to achieve his own American Dream.
Not only had Willy failed at being well-liked, rich, or even a relatively alright salesman, he also may have failed as a father. The reason for it is because he became more and more aware that his inefficiency and lack of reality has passed onto his kids. Happy is, like his name implies, a "happy-go-lucky" kind of guy who lacks character and foundation. Biff, similarly lacks an aim in life and has just discovered the joke that his life has been; a mere repetition of Willy Loman's failed dreams.
Willy is now in his 60's, and has come full circle with his life, realizing that it has amounted to nothing. Hence, his metaphorical action of planting seeds represents his last minute frenzy to, somehow, try and leave something behind.
I think that the idea is that Willy always intended to start a garden but never planted seeds before. He has waited until it was too late. Now the little house is surrounded by high-rise buildings that shut out the sunlight. One of the themes of the play is that a person should prepare for the future. Biff is a failure because he never prepared for anything. He believed in Willy's philosophy that the important thing in life was to be well liked by other people. In this competitive and rapidly changing world, other people do not really care much about one another. Under capitalism it is a matter of survival of the fittest. But that doesn't necessarily mean that some other system would be any better. There are probably always winners and losers in every system. Charley, the realist, tells Willy:
The only thing you got in this world is what you can sell. And the funny thing is that you're a salesman, and you don't know that.