Roger Clemens - 1999 Upper Deck Black Diamond #88 (#d/1500) (Blue Jays)

2 reasons auctions are popular way to sell vintage sports cards.
#1 Desirable items tend to get top dollar (or better !)
#2 Everything goes
... This can be good - Everything gone, happy with final total
... or can be very bad - Everything gone, but at super low prices

OTHER WAYS TO SELL YOUR CARDS
•eBay Buy-it-Now •card store •swap meet •Craigslist •garage/yard sale
or •DONATE to a charity for tax write-off
Not selling but perhaps easiest with possible $$$ return.
ASSUMPTIONS:
#1) You are one of the RARE tax payers left in America
#2) You have mostly late 80's & 90's "junk"
Consider donating the "junk" and keep better stuff to sell later. Tax deduction was based on LESSER of "what-it-cost" & "What-it's-Worth". For "What-it's-Worth", I use Beckett which can be higher than what you paid. Check with your tax guy.

Cut/Paste TurboTax discussion link below:
https://ttlc.intuit.com/questions/3372284-are-trading-card-donations-deductible-if-so-how-much Possible Charities for Donations
•Goodwill   •Salvation Army   •Cerebal Palsy   •Children's Hospitals   •Cubs scout troops
...or an online charity
  •Cards 2 Kids   •Commons4Kids.org   •CollectiblesWithCauses.org
Verify all "charities " before donating.