The economic argument is one of the strongest reasons to be a "strict" vegan; a strict vegan does not use any animal products, whether found in food, clothing, or other consumables. It is based on the general ethical responsibility argument:
By paying for animal products, you accept the full responsibility for the life and death of the animal that produced them. By paying for animal products, you are paying the farmer to grow the animals for you, and kill them for you. By demanding cheap animal products, you are demanding the farmer minimize costs -- no matter how much the animal suffers as a result. If you accept animal products from other people (such as through a free dinner) you are also complicit, because you are creating a demand for the animals to be produced. If you cannot accept this responsibility, you must stop using or buying animal products in order to retain the courage of your convictions.
This includes animal products in which the animal does not die as a result of collecting its secretions. Although the chicken may not have died to produce the egg, she had to suffer in a cramped cage for 24 hours with 5 other chickens. After a few hundred eggs, she is killed; so there is partial responsibility for its death; also, for every female chicken, a male chick was killed at birth. Although proper milking equipment does not cause a cow to suffer beyond a mild irritation, the cow had to be kept pregnant most of the year to give milk; the babies are raised in crates in horrible conditions to become veal calves, and the cow is turned into hamburger when she is several years old. By buying eggs or milk, there is economic complicity; without your money, the farmer would not kill or torture the animals.
There are a few areas in which it is not clear whether you are complicit. For example, new leather or silk clothing is clearly economically linked to the original animal, but used leather or silk clothing isn't. However, by paying market value for the used clothing, you allow the seller of the clothing to collect back some of the original money they paid for the item. A subsidy of the original purchase is partial complicity. The lower the price, the less complicity! If the seller is selling the item just to get rid of it and is selling it for way below market value, then the complicity is very small.
By purchasing or using a used leather jacket, you decrease the supply of used leather jackets, and increase the price. Someone who might consider a used leather jacket might buy a new one instead! However, recovering animal-related items that would otherwise be thrown out does not cause complicity.
Cruelty-free leather is produced in India by one of the lowest castes from the skins of dead animals. This leather is used by most Indians. In this case, the complicity is for the suffering of the individual people who make the leather, and not of the animals! The bodies of animals who die a natural death can ethically be used by humans, providing humans did nothing that contributed to that animal's death, such as pesticide poisoning or habitat loss.