Question about copyright for vintage images



Hi people, I’d like to start making auction templates for ebay to sell. But a few things in mind, and need your heip.
Does anyone know of any sites that you can find graphics, backgrounds, fonts, etc. for commercial use? I am confused a little about using vintage graphics. I’ve seen many people selling vintage graphics. If they were made before the copyright laws and are considered public domain, how are people able to sell these and put their own copyright on them? Aren't they public images? I know people get them from greeting cards etc. What if you had the same greeting card (because I am sure more than one was printed), can you use the image? For example, a painting of a rose. I feel like this is such a general image.
Also, what about basic stuff like dots or stripes, there is nothing original about them.
Hopefully someone could clear this up for me or direct me to a good site or book to find the info. I have been searching for months and everything seems so ambiguous