What is the name of your state (only U.S. law)? California
Is it legal to use stock images of items from marketplaces (i.e. Amazon, Macy.com, third party sites) you find online for your own website?
If so, under what conditions? ...
Nothing you have said indicates your use of copyrighted images on your website would be a fair use of the images. Fair use is not permission to use copyrighted material. It is a defense to infringement when you are sued for using copyrighted works without permission.
Most of what you find online is rights-protected material. There is an owner somewhere who holds the rights. To use images you find online, therefore, you must locate the holder of the rights for permission to use their material for your website. This permission is usually granted in the form of a "license."
The terms of a license can vary depending on the owner of the work and the work that is to be used. Some licenses will grant
limited nonexclusive rights to use the copyrighted material, and some licenses will grant
exclusive rights to some of the copyrighted material. And all copyrights in a work can be transferred by the original copyright holder to another, creating a new copyright holder, this through a written and signed transfer agreement.
There are some works that are now in the
public domain and can be used by anyone freely. But public domain has a specific meaning in copyright law. It does not just mean that the materials are publicly available but rather that the materials have either lost their copyrights due to the passage of time (for example, works created prior to 1923 are now in the public domain), or through a failure on the part of a copyright holder to renew a copyright in a work published between 1923 and 1963, or because the work was published prior to 1978 and lacked a proper copyright notice that was never restored ... and there are other reasons why a work may now be in the public domain.
The odds are pretty good that what you found online is NOT in the public domain. The odds are pretty good that there is a copyright holder somewhere expecting to be compensated for the use of their work. It is up to you to locate the copyright holder and get permission. Using a copyrighted work
without authorization from the copyright holder can result in the work being removed from your site via a DMCA takedown notice, and a demand for compensation, and an infringement lawsuit. If a copyrighted work has been registered with the US Copyright Office prior to infringement, the copyright holder can be awarded statutory damages from $750 to $150,000 per infringed work (the amount depends on the work, the infringement and the infringer).
You might want to create your own works for your website. This will make your site unique among others and this will also help you avoid the problems that can come from using the works of others.