When collecting older mint sets, it is easy to overestimate or underestimate the value of a particular set. Keep a guide book with you when buying or selling coins. (Also, make sure your book is up to date. You wouldn't walk into a coin shop with a 1984 Red Book and ask to buy a $100 coin for $17.) Small local coin shops are great places to buy the cheaper proof sets and such. Often you can score a $8-$10 set for $3-$6.     Don't forget the rare sets. For example, make sure to examine all 1981 proof sets closely. A type one is worth $5, while a type two is worth over $270! Some dealers may forget to double check for such rarities, so you could really get some bang for your buck!