This word is derived from a misspelling of "Deceiver". It has risen in popularity with the rise and prominence of Facebook's Marketplace.
A Deceder is someone who constantly lowballs people but also lies about market price.
Usually a noun, it can also occasionally be used as an adjective.
I knew my comic was worth $100, and this Deceder guy said he'd give me 70% of value, but then when we met up in person he claimed it was only worth $80, and tried to pay me $56. Just a huge waste of time.
That guy was such a Deceder, you'd think he was from Austin TX, home of the lowball king, and "Following for Price Drop."
Denedy is a gorgeous girl. Loves to make the people around her laugh, loves her friends, will do anything for them, but betray her and beware. She is a little hard headed but a sweetheart.