I own a lot of graded cards, and have learned a lot over the years. Here are a few ideas that I think are key, from the perspective of a collector, and not a reseller:
1.) Identify what type of graded card collector you want to be. For example, I primarily buy cards that are already graded, rather than sending in cards from my personal collection.
2.) Don't waste time re-grading cards.
3.) How to buff out scratches on graded cases (recommend the Novus No. 1 and No. 2 Products)
4.) Only use the top 3 grading companies, PSA, BGS/BVG, SGC
5.) Concentrate on what your grading limits are, to avoid an inconsistent graded card collection. For me, anything from the 60's I look for 7's or better, anything from the 70's, 8's or better, the 80's, 9's or better, and finally the 2000's, I will typically look for a grade of 10. Any cards that are older than the 60's era, is pretty open, that is really the only exception in my book.
6.) Know the market, to avoid over paying. I use three sources, eBay (perhaps the most important gauge), PSA Price Guide, and the Beckett Price Guide.
7.) Realize your financial limitations, and know that there is nothing wrong with purchasing un-graded cards. One can always decide to have their raw cards graded later on.
8.) Paying a premium for graded cards, or being obsessive about sending cards in for the hope of a high grade, isn't for everyone.
9.) Watch YouTube tutorials on how to submit graded cards from the big 3 companies, if that's the goal.
10.) Have fun.
- Joeyd