I usually go and have a mooch about. Its quite big and that doesn't always give punters enough time to hunt about. However, if you take a stand, you get in early and in my case, I used to sell half my stuff to other trade stalls before the punters got in. Once I sold a trailer load of kit while queuing to get in! (And I sold the trailer!).
Don't go with huge expectations of making enough to retire to Monaco, but a small stand with some useful bits will usually do well. Word of advice, work out roughly what you want for what you are selling before you get there and don't go cheap. You don't need to if what you have is good and has value. If not, some other seller will come along, buy it and put on their stall and make a profit.
The indoor section is quite big and is usually taken up mainly by trade stalls and the usual cheapo tools people and so on. There is space for private stalls and if the weather turns, you are onto a winner.
Go with the mindset of flogging a bit of kit, making a few £££s and having a bit of fun with some interesting people. Don't go there expecting to make vast amounts of money unless you are a proper trader - its not worth the grief. Treat it as a social event and a glorified car boot sale for Land Rovers