Only meaning I know is a guy who carries out a lot of different tasks, like in a job description, "wanted person for General Cleaning/dog walking/daily shopping). The description comes from the forward slash between the words. The tasks, of course, may not always be entirely legal.
This is an interesting thread!
Given that definition, perhaps "slashers 20 a yap" could mean "Will do various tasks, $20 each"? Yap is slang for talk, so maybe "Will do various tasks, $20 each, talk to me if interested"?
It could also be seeking instead of offering
My guess, could be completely wrong