I'll try my best to weigh in well because I was recently in the exact same boat as you and i'll mention a few things:
Wem, for your budget, gumtree is probably going to be your friend. Heaps of good deals on good yaks go up daily, sometimes with extras like PFD's, sounders, anchors etc. I've seen wavedance kingfishers and ocean kayak prowlers go up for like 350-500 over the last month or so in good condition, as well as the host of other ones for similar prices, and hobies going into the 1000-2000 range. Few good deals pop up on facebook marketplace and kayaking facebook groups too.
For your budget of 500 bucks i'd avoid buying a new kayak, because most of what you are gonna get are cheapo pool toys like whats made by kayaks2fish and other weird brands. 4liters already said this, but they're mass produced kayaks likely off alibaba for 99 bucks a piece, branded and then repackaged and sold to you. They also have that weird sales tactic of pretending they're on sale all the time when they're not. These kayaks are too small and seem to me like they are designed to coax in new kayakers looking for a cheap yak. I'm also still signed up to gumtree alerts, the number of these things that pop up every day goes to show how people buy them, realise they're bad yaks and then resell them a few weeks after purchasing. I would brands like this like the plague if I were you.
Like cheaterparts said length has strong correlation with safety and stability, ideally you want a yak thats 4m or more for the bay. Also worth considering visibility, I deliberately bought a bright yellow yak so that I can be seen easily. I wouldn't go buying blue/deep green/black yaks because its harder for people to spot you on the water. Weight is also a thing to consider, yaks that weigh 30kgs+ are not only going to suck to transport but will be slow, and make it harder for you to get places quick when you need to.
A lot of these cheapo kayaks come with a gimmicky 'stadium seat', which is just mimicking a hobie's seat. All this does is cause the yak to be more unstable, consequentally making it wider, which makes it heaveir and slower. Not something you want.
This became a bit of a buyer beware when coming to buying a kayak, but if you wanna look into some more advice on what models to pick based on specific factors, you should read this thread -
http://www.fishing-victoria.com/viewto ... ilit=kayak
I posted it a while back while I was looking for a yak and heaps of helpful advice was posted that will probably help your decision too.
cheers.