A friend of mine told me a story a few years back...
Back in the early '90s, he and his wife paid $1,000 to attend a private signing of Star Wars actors in L.A.
According to him, -everyone- was there: Ford, Hamill, Fisher, Guinness, Cushing, etc.
And that price got you one autograph from each person.
So, he took his original Star Wars poster and had it signed by everyone.
He got it framed but later sold it to help pay bills.
And now he wishes he never did.
So it's possible to get a bunch of signatures on one item, but the entire Infinity Wars cast? Dubious.
My mantra is: if I don't personally see it being signed, it's probably fake.
On occasion on ebay I'll see some limited edition Star Wars posters that were pre-signed and given to attendees of the premieres, but wonder if the stars actually did sign them.