One reason to learn to grade for yourself. Professional slabbing is a
good idea for key and semi-key date coins, or if there's a possibility
of a counterfeit or altered coin (e.g., a 1916-D Mercury dime). If a
common date coin has the possibility of attaining a super-high grade
(e.g, a current Shield cent that could grade MS-69/MS-70) that's
another consideration.
The 1970-D Kennedy half is the key to the series, and they can be had
for under $30 in MS-64, or even less in a lower grade. At MS-65
they're worth about $70, in MS-66 $285, and in MS-67 $4,500, the
highest grade shown in the PCGS price guide. If you opened a 1970
Mint set (the only way to get a 1970-D Kennedy back then), you
shouldn't assume that just because it's in an unopened package that
it's worth big bucks. Grade it yourself. If you think it's an MS-65
or higher, then consider having it professionally graded. If you
think it's an MS-64 or lower, it isn't worth it.
As for selling a slabbed coin on ebay for $10, it's basically a crap
shoot. Using my 1970-D Kennedy as an example, someone thought their
half was worth more than the slabbing fee (last I knew was about $30).
and it came back an MS-63, worth about $10.
Jerry