Items are generally worth a common or uncommon from the year they were released. Older items may be worth more, here's a guide on the values of some older items:
Forum/viewtopic.php?f=20&t=559754Store items that can be bought with C$ are always worth that amount in C$ (or the equivalent in pets) unless they're retired from the store, and items that are free in the store are generally worth a common/very common.