HESS TRUCKSThere are certain collectibles whose increasing popularity killed their potential value. Comic books suffered this fate within the past few decades as the number of titles and editions available outstripped collector demand -- leaving old 10-cent titles with the most inherent value.

The same holds true for East Coast gasoline company Hess' (HES) toy trucks. The company began selling plastic versions of its tanker trucks in 1964, but as the toys became more popular in the 1980s and production increased to include not only more trucks, but minitrucks, resale prices plummeted.

"Early Hess trucks are good, if you have them from the early '60s and 1970s," Kahn says. "If you have them from the '80s, '90s and on, they're a flea market item."

An original 1964 truck recently sold for $1,500 on eBay, but versions of the same truck have sold for $620 or even $300 without the box. With the exception of a rare in-box 1993 truck that sold for $500, the overwhelming majority of Hess trucks sold after the 1970s resell for less than $50. While not a ripoff, it's not a great investment.

"I tell these people that they should sell their collection and put it in gold or put it in the bank and get 1.5% a year," Kahn says. "It beats losing money every year."

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