A few quick questions, hopefully you can help...
1. what is the value of a Cal Ripkin Jr RC
Answer: It depends on which of his rookie cards and the condition. You've got Topps, Topps Traded, Donruss & Fleer to choose from. Also can depend on graded or raw.
2. same question but for a Rickie Henderson RC
Answer: Beckett values a Henderson rookie as high as $70. Prices will vary by condition either graded or raw.
3. what is the foremost industry price guide...is it still Beckett?
Answer: Beckett is still seen to be reliable although many folks quote SMR (Sports Market Report). Surely Beckett is good enough to get a decent idea.
4. is ebay the best place to unload cards?
Answer: EBay is probably the easiest place to unload cards and using their search feature you are able to see approx. value in the open market for your cards.
Hope this helps.