Help - fcbk advertsiment

Guys, i need some help.
Well, Im producer and work with some artists. Lately, ive received a lot of emails from people/agencies asking me about { Hello, I’m the member of X advertising, are you open for paid advertising on your facebook page? }
They are offering me daily post and a payment weekly. Anyone here can help me with that, cause i have no idea if its safe to do it. There is no contract, just make a post per day during a month and getting weekly payment.
At least i have 7 people/agency offering me this ‘‘job’’.

Massive red flag right there

What is redflag ?
This is real ?