I had a full-blown business sellin cards on eBay up until a couple years ago... started off just sellin parts of my collection I didnt want anymore (I had accumulated over 1 million cards, mostly football & baseball). Then I started flippin cards, buyin em for cheap as shit & sellin em for 2-4 X profit on eBay. Ended up with like 2000 feedback, but it got to the point where I hit a wall & couldnt pay my damn eBay fee (it didnt help they kept raisin the listing fees & shit).. so I ended up with a $600 eBay bill I didnt pay until last year..
but yea, if you buy a Beckett to get an idea of what your cards are worth, you might as well divide the Beckett price by 7 because that would be the actual value... at most pretty much every card sells for 15% or less Beckett on eBay... unless its 1910's-1960's star baseball cards which still command premium, or the UPPER TIER rookies from the past 30 years / autos / serial numbered inserts... those still go for like half Beckett or more depending on what player / card it is.
I still have about 10,000 cards that meant the most to me... 4 T206 from 1909-1911 in good condition, hundreds of autos (Mays, Killebrew, Aaron, Favre, Manning, Payton).. couple thousand rookies.. Hank Aaron 3rd year Topps in good shape, Killebrew rookie... yea. Cards like those I don't see dropping significantly in value, altho they probably wont increase significantly either. Somethin to give the kids when they're a lil older.