This really (thank goodness) isn't anything that I worry about,
since I've been living on my own for a while--but--my parents
collect antiques, and my dad is huge into stocks, so I just
try to talk to them about it from that point of view.  For example,
I have Ryoko cels that I bought a year ago...I would never sell them,
but I can show my parents what similar cels are going for on ebay and tell
them what I paid for them, and my dad thinks its a great investment (!--he even bought me a $150  cel for my birthday last year, and I got 2 cels for Christmas).
The only drawback is that now he tries to make me interested in 
stocks.
The only trick is, only tell them about the ones that were 'good'
deals!  I have some I would *never* tell my parents what I spent on
them...which is sometimes an indication that I spent too much on
the cel. Actually, I'm dreading coming up with an estimate of what
I should insure my collection for...I think it's going to make
my stomach hurt!
rosemary