The problem with word of mouth is that social media allows for anyone to open their mouth. This recent thread on a fake snap pikachu card is a great real life example. Luckily this site is full of seasoned collectors. That same card was posted on a social media platform, and majority of the individuals thought it was real or were naive.
Any item of significant value needs to be psa graded. This is the standard for all auction houses, and necessary for today’s market.