All the agents are freight forwarders, but in the context this term is usually used in, it refers to someone who does the job of a shopping agent, but as a private party. Essentially a shopping agent without a business license.
These people that use their apartment as a warehouse and have no employees. Consequently, they have a smaller overhead and less risks, so they can offer their service for cheaper.
The main disadvantage of using them is that it's very easy to get scammed, reputable shopping agents have their reputation to lose, these private middle men can just disappear. Another disadvantage is the unreliable quality control: They don't have a set procedure to inspect items or when and how they will forward them to you. Shopping agents utilize economies of scale with trained employees and high tech equipment. One of the biggest financial risks is their lack of experience/data to give you a quote for the international shipping. Since they don't want to risk losing money they will quote you more which leads to you ending up paying more.
Contacts (usually WeChat) for these freight forwarders can be found on reddit, or you can buy their service on AliExpress.
Every now and then the community decides it's cool to leave the "greedy" shopping agents and to use freight forwarders. So far they always went back to using the regular shopping agents.
I do not recommend using these services for beginners, and even experts should be careful. It makes no sense to take a total loss (-100%) in order to save 5%.