The trouble with a lot of these freelancer websites is that people don't want to pay a decent rate, especially to newcomers, and some don't want to pay at all, so you will spend a lot of time chasing money and not getting it
Not true with People Per Hour at least. I've been on there for about 5 years, I rarely bid for work any more as I have a few clients I work with regularly and who approach me with work. I have built up a good relationship with them and there is a level of trust there. On PPH you can have money paid upfront. This is then held in escrow by the site and when the job is complete, you send the invoice and the money is released. I have only ever had one client go rogue on me - work was done by me and uploaded as promised, he completely vanished and wouldn't respond to emails or messages and ignores invoiced. It took about a month, but because I'd asked for payment up front, I got my cash.
It's also true that there are a lot of crappy sellers and crappy buyers too. If you see an advert stating "You right me good content" you just scroll on by as it's clear the buyer isn't serious. It's not hard to work out which clients are serious - they give you a proper brief and set a realistic budget. All of the clients I work with are UK based - they want another UK based person to work with, someone who knows the local market, can write in British and not US English, and who has shit hot spelling and grammar. They are willing to pay more for quality. I will not work for buyers from the Indian subcontinent, the Caribbean or Africa - they are used to writers who charge $2 or less for 500 words and expect that sort of pricing. Quantity over quality.
Having said all that, all of these freelancing websites are about reputation. I have 5 years worth of positive feedback from other clients for people to look through. They know when they are looking at my profile that I will do what I say and deliver on time, and they have hundreds of testimonials saying just that. If you're new, getting that first job or two is the hardest part and yes I do think it's worth bidding very low on a couple of small projects to get that feedback and reputation. It's a strategy which worked for me.
Earnings can vary hugely on PPH but it depends what you want out of it. I want to work 8 to 10 hours per week as a maximum and at a time which suits me, and the freelancing sites allow me to do that.