Depends on how old. The cards in the 80's were worth way more in the 80's. Cards from the 20's, yeah, they generally increase. It's the old supply and demand concept. Prices increase when demand goes up or supply goes down. 20's cards have low supply. 80's cards had high demand in the 80's cause everyone thought they would get rich. Now, the supply is so high and demand went down, so late 80's/90's are generally worthless.
As for the pricing on the site, I like the feature and use it as a guide, but not as a religion. Someone reporting a price doesnt make your card worth more or less. If enough people report prices, the concept is the price median would somewhat resemble the market, but the reality is there isn't enough data loaded in the site to be accurate. If I see a card is worth a lot on the site, I don't think I have anything yet, but I'll go check eBay to see what recent auctions have gone for. If you use the site's pricing in this manner, it is an awesome feature. If you try to use it as gospel, you really are setting yourself up for disappointment and frustration.