Why would anyone greatly fear someone taking vengeance on them ("anyone who finds me will kill me") if, as some assume, no one else but Adam and Eve where living at the time? The Bible does not state Adam's age when either of his first two sons were born. Seth was born after the death of Abel when Adam was 130 years old. The youngest age at which the first patriarchs (Seth, Enosh, Cainan, and all those to Noah) gave birth to their first son was sixty-five.
If we conservatively assume Adam had his first son at 65 then that would give Adam and Eve 50 years or more to procreate until Abel's murder. A footnote in the writings of the Jewish historian Flavius Josephus states a very old tradition that said Adam had a total of thirty-three sons and twenty-one daughters. The number of Adam and Eve's children, grandchildren, great grandchildren, etc. that existed when Cain received his punishment would have been enough to warrant his concern.
Why did God issue a stern warning that if Cain is met by someone they should not kill him? It is because, in part, it would defeat God's judgment and the purpose why he was allowed the world's first murderer to live.
The opinions and speculations about what was the mark have varied greatly throughout the years. Different people and groups have argued that the special sign God gave was a horn (either short or long), leprosy, Abel's guard dog, a special symbol or letter, or a letter used in God's name, a tattoo, dark skin (some feel he was turned black), His own name and even the rite of circumcision.
The truth is, the Bible simply does not clearly tell us what was the unique mark of Cain. Any conjecture, therefore, regarding this issue is ultimately based on opinion and not the word of God.