Nick777vvv, welcome to the US forum! There is a lot of discussion here about stamp values and prices and such. In my humble opinion, catalog values and even records of auction or dealer sales for a given stamp# are secondary to my gut attraction to a stamp. Some of my favorite stamps in my collection would get no better than fair reviews if expertized. For myself, appreciation of a stamp's beauty is not tied to the market value (although, obviously if it is a rare stamp that makes it more exciting).
It sounds like you want high quality (emphasis on centering) unused stamps. I think you can find plenty of US Classics, especially on sites like
ebay, that would meet your criteria and have not been graded or otherwise snatched up by professional dealers/auction houses. If you want to limit yourself to those with original gum and no gum faults, the selection will be much more sparse. But, if you are willing to accept "minor faults" like small gum issues, etc, then you will have plenty to search for.
I'd recommend knowing the types that get faked (
http://www.stampsmarter.com/1847usa/index.htm is a great resource), especially Washington Franklins. If you avoid stamps with certificates, I think the major issues you will run into will be:
1. Gum faults not clearly shown or described
2. Incorrect color identification (e.g. calling a #65 Rose stamp a #64 Pink stamp)
3. Incorrect grade identification (e.g. calling it mint when it is just unused, or Fine centering when it is just AVG centering)
4. Reperforated stamps
Hope that helps. Please keep asking your questions!