Two things are worth emphasising at this point. First, it is not to be expected that you will have any firm grasp on their nature or operation. The beginnings of understading of wabi and sabi will come only as you read, enjoy and study Genji. You have been presented with the accrual of æsthetic and intellectual concepts gradually in order that you will be able to appreciate and increasingly recognise their presence as your reading of Genji progresses.
Keep in mind, in this regard, the twin points repeatedly made in lecture that Genji is not a novel designed to fictionalise wabi and sabi and that our task is not to pick out the sections of the author's literary enpuzzlement. Quite the contrary, part of Murasaki Shikibu's superlative genius is revealed in the fact that her artistic sensibility artistry comprehended the many large and unformed ideas and assumptions behind the Heian way of experiencing the world and encoded them in the original literary achievement that is Genji. It was through reading of Shikibu's masterpiece that Japanese artists, critics and scholars in later centuries came to develop and elaborate the notions that became wabi and sabi.
I have come across an excellent book online through our SFU Library which is a collction of essays from various scholars of Japan, entitled Japanese Aesthetics and Culture : A Reader and edited by Nancy Hume. The link, requiring authentication to the SFU proxy server, is here.
The second point to note is that, by the nature of things, wabi and sabi have amorphous boundaries and are fertile of definition and explanation. As we have been learning, the Japanese mind resists systematisation, and is vexed by no shinto hobgoblin of a foolish consistency. You must rid your mind, grasshopper, of a need to find limiting safety in fixed definition and tight agreement along your path to enlightenment in wabi and sabi.
My own interpretations of the concepts are quite centric, and are those which, I believe, make them most readily accessible to Western students, without in any way sending them off on any misleading directions or falsifying by over-simplification. I have been informed in this, most obviously, by the modern great D.T. Suzuki, whose Zen & Japanese Culture will be be found on our course reserve. As proof, consider this quotation on sabi-ness from one of the authors collected in the reader linked above:
Towards the end of the medieval period another aesthetic ideal, that of sabi, joined yugen. Sabi was a very old word, found as far back as the Manyoshu where it has the meaning of "to be desolate." It later acquired the meaning of "to grow old" and it is related to the word "to grow rusty." In Tale of the Heike we find it used in the sentence, "It was a place old with moss-covered boulders, and he thought it would be pleasant to live there." It seems likely that already by this time (the thirteenth century)sabi suggested not only "old" but the taking of pleasure in that which was old, faded, or lonely. To achieve the end of yugen, art had sometimes been stripped of its color and glitter lest these externals distract; a bowl of highly polished silver reflects more than it suggests, but one of oxidized silver has the mysterious beauty of stillness, as Seami realized when he used for stillness the simile of snow piling in a silver bowl. Or one may prize such a bowl for the tarnished quality itself, for its oldness, for its imperfection, and this is the point where we feel sabi. If the Noh is the highest expression of yugen, sabi is most profoundly felt in the tea ceremony, and to attend one even today is to get a glimpse of sabi at its purest. The tea hut is extremely bare and almost devoid of color. If a flower is arranged in a vase, it is usually a single, small blossom of some quiet hue or white. The tea utensils are not of exquisite porcelain but of coarse pottery, often a dull brown or black and imperfectly formed. The kettle may be a little rusty.