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warshawlawgrowth items: prewar and 19th century boxing. We've seen strong price growth this year but I think there is much more upside potential, esp. with high grade cards in slabs.
overpriced items: E cards.
underpriced items: postcards. some real bargains out there.
Stuff that sells for way too much: 9 and 10 slabbed postwar cards, esp. 1970's cards. I've gotten a few back from SGC in 9; the difference from 8 to 9 to 10 is negligible and certainly not worth the premium paid. I recall just a few years ago that 8's were touted by the auctioneers as the primo cards, until enough collectors had 8's and could compete with auctioneers on ebay. Then all of a sudden it is the 9 and 10 that are the primo investment cards. It is bull****, same as stock speculation in internet companies was.
ebay safety: read the feedback on the seller of anything expensive; if the item is expensive and you cannot take the loss only use a seller with paypal or credit card potential so you can protest if there is a problem; get all over a seller who is late with an item and do not hesitate to apply for a refund from paypal and/or protest a credit card charge, because the seller can always make good; stay away from high end cards in raw form from amateur, unknown sellers unless they are local to you (so you can take them to court and/or take them apart if they cheat you); never bid in an auction with a seller who has private feedback or a private auction; never bid on an item where the seller invokes the name of God (esp. God bless you in the listing--a sure sign of a cheat 
); never bid on an item where the seller says it may be fake--it is fake; always opt for postal insurance on expensive items because it forces the seller to create a record of shipping something when he goes to the post office to drop it off; never bid on an item if there are problems of any sort, walk away and don't bid.