Even if YL do not endorse or take responsibility for transaction across its platform, YL states that the overwhelming majority of its users are trustworthy and well-meaning. YL is doing its best to verify users and possibly, the existence and the quality of products/services presented in here. But, as with all transactions, please practice caution when interacting with others. Keep it transactional, avoid taking it to a personal or relational level.
For example, prefer local seller/buyer transaction, and avoid online or remote transactions. Look out to meet in person with seller/buyer themselves, refuse to meet with an intermediary person. Since, meeting in person may also come with some downsides, use these extra precautions:
Insist on a public meeting place like a bank or police department. Some police departments have buy/sell areas set aside for meeting.
Do not meet in a secluded place or invite strangers into your home.
Be especially careful buying/selling high value items.
Tell a friend or family member where you're going.
Regarding service announcements, be very careful, do not trust blindly the nice words and promises of the ads, but do your search first. It is your responsibility to verify the legality (including licenses and authorizations), competence and quality of the promised services before accepting or committing. YL is just an announcement channel and disclaims any additional liability.