Beckett is helpful if you're getting back into the hobby, used as a guide along side eBay which is so large it more accurately reflects market value - or what people will pay.
Recent example Jeremy Lin - people were paying $1000 for a card with a 100hi BV , Beckett mag and sometimes online guide can fall behind and aren't regularly updated - eg 08-09 Precious Metal Gems , only last week did Beckett online update (increase) prices for them, after all the hype and sales. Prior to that you had a Ray Allen red sell for 360 which BV was 60, now did they pay too much? No, that's what people wanted to pay, that's what it's WORTH. I paid 180 for a KG Graded Red which BV50-125 ungraded, and I'm freakin stoked, I think I got a bargain, love the card.
exquisite, UD Black, some players and singles, heaps of stuff sell for more than they may be in Beckett for, the market is so varying and volatile you need to use all the information, together with what you are personally willing to pay for something and figure it out.
A reverse example. You look up an Avery Bradley auto, Beckett Val is 10 so you list it for sale for 8 which seems fair, and wonder why it sells in 1 minute, well, he's had a great month, played awesome, more people want to invest or get a RC, so in that time now they're selling for 50. Don't assume, don't screw yourself or be screwed, do your research.
Hope some of this babble makes sense.