Is it permissible to purchase computer software from a website that sells it at a lower price than the producing companies, relying on buying it from wholesale market customers, whether it be old software that companies have ceased producing or modern software?

It is permissible to purchase software from the website if the website acquired it through legitimate means, such as buying directly from the producing companies or from someone who genuinely owns it.

However, if the website purchased it from someone who obtained it through illicit means, such as piracy, then it is not permissible to buy it from them. That which is inherently unlawful cannot be made lawful through purchase. If you know that someone has stolen money or usurped it, it is not permissible to take it from them.

The fact that the software is cheap or that the producing companies have stopped manufacturing it does not change the ruling. The decisive factor is whether the seller acquired it through legitimate means.