Thank you all for the advices, they are all very good!
To add some of my observations:
1. I am also a little bit skeptical about using agent for rental, reason being no matter how long it takes to rent out, as long as it's rented, they get paid the same commission. So even your agent is busy with buying and selling, your rental might be lower priority for him/her.
2. The application and the credit check are definitely value added by the agent, however, you can also hire an independent company to run background check.
3. I ended up paying the Facebook marketplace ad myself, as I can't stand it's being vacant for long time. I also went thru all the websites, if they don't automatically pull ads from MLS, I will put the ads there.
4. But I would say this is useful learning experience, as I've learned where I made mistakes and how I can avoid them next time. For instance, I use two months vacancy rate when calculating cash-on-cash return, I thought I am very conservative, but it's actually not.
Thank you all for good advices!