An "amateur" can make money but I have found the people who really enjoy coins end up doing better than people who are only in it for the money.
There are three major risks:
1. Not getting what you paid for. Grading is everything! But even professionally graded coins have high-end and low-end coins for the grade. If you buy low end coins, they can be tough to sell. How do you avoid that? Do business with the right dealer who will not sell you crap. Now if you buy stuff right from the mint, that is less of a concern, but you are limited in what you can buy. And if you buy "raw" coins (not graded by a professional service), you are really playing with fire unless you know what you're doing.
2. Paying too much. Even if you buy a properly graded coin, if you pay too much it will take a long time until you can sell for a profit.
3. Not getting full value when selling. If you sell to a dealer, you know you're not going to get as much as if you sold to another collector - the dealer has to make a profit to stay in business. If you can take good photos and list coins on ebay with good, honest descriptions, you can minimize this problem.
The place to start is to read "Coin Collector's Survival Manual" by Scott Travers.