Copyrights is something to always be aware of. I personally only purchase products that will allow me to have re-sell rights, otherwise I don't even bother myself. If I really see something I like, I just create my own version with my own twist of it from scratch, then it's mine.
Depending on what kinds of items you are looking for, an example of a good resource to start is Dover Publications, you can use up to 10 of their items per creative idea for resale without getting permission. I make alot of my own items and plan to open my own digital store front offering my custom made items for commerical use to others for resale.
When I do write to companies for permission if I am unsure if I can use something of their's, I let them know how I am using it, get their permission with any guidelines and save the permission in my records, (like from Dover, I wrote to them for clarity because it was worded confusing on the back of their books.)
good luck, my motto, if in doubt, don't use it if you want to re-sell it. Try to only purchase items that specifically say you have resale rights.