Itch.io is a bit of hit and miss. I've noticed that people offering money for the job get inundated with responses, and people offer to do it for less to get the job. The majority have people contact them on Discord to continue a conversation - but sometimes the conversations from Discord filter back to here. A case was where someone had recruited a team, and offered to pay a graphic artist, but then killed the project. The graphic artist had already done the work...

Payment is between you and the person you're hiring. It's difficult on a platform like Itch.io, because you both have to trust each other with nothing guaranteeing that trust.  If you go to a site like Fiverr or Upwork, you have some protection. 

Where you might have some success is that when you browse the assets, there are developers that have posted free or paid assets but also offer their services for custom jobs. Perhaps search for the kind of asset you want, and look at the pages that come up in the search results.

Hello! I can't give you the perspective of someone that has hired people from this page, but I've actually established contact with a bunch of different people that have paid me for my services and come out satisfied with my work. And yes, I think that like the previous comment said, it mostly revolves around trust. Here on itch you don't really get to have a lot of references on the work ethic of the people you might hire here. In my experience, it helps out a lot for people looking for services to check out the artist/developer socials or itch page in order to know if they are actively doing work and are able to deliver what you're asking for. Like, if you get a response from a dev that wants to work for you, but has no portfolio nor has been working on projects recently, it usually indicates a pretty red flag.

In my case, I always promote my services as a pixel artist and animator while showing my most recent work and always linking my portfolio. Recently I've been thinking of updating my website to have reviews of people that have hired my services. I guess what I can say is: be smart about this. It's true Itch contains the most accesible and affordable services for indie games (or even names that come from studios), but there are also a lot of people that don't really know what they are doing. So my advice would be: Research the one that is asking you for the work and see if they are actually putting in effort to sell themselves. If you see they have a good ammount of work under their belt and are constantly looking out for work opportunities, my guess is that they wouldn't risk getting exposed as a scammer.