1969 O-Pee-Chee/OPC # 31 Mets Rookies (Gary Gentry/Amos Otis)

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.

Team signed baseballs were the thing well before single-signed balls exploded on the market.
What is a "Team Signed Baseball" ???
Simple answer: A ball with XXX signatures of a certain team for a certain year. What is difficult is the XXX. Baseball tons of roster moves make it nearly impossible to "Get Them All".

Generally, team signed baseballs from early 1900's had 10 to 15 signatures, the 1940's that jumped to 18 to 25. Joyce Sports Research Collection (Notre Dame) says "signatures must include only members of a specific team from a specific year, and there must be some approximation of completeness."

Not concrete but to me a "team ball" MUST have ALL the team's STARS (unless a rookie or in season trade) and in today's market at least 20, preferably more, and the manager.

Determining Age of Team Signed Balls
"Official" league balls have stamped signatures of current league presidents on the "sweet spot". Starting 1934/1935 balls were produced by Spalding (NL) and Reach (AL). Rawlings took over in 1977/78. Have a possible team roster at hand, ESPN & baseball-reference.com have great sites), decipher a few signatures then solve the puzzle.