Sites like Banggood (and a many others) allow you to pay in various different currencies. They will have a "native" currency which they set their prices with and then other currencies are converted by them using a less than optimal rate.
It turns out they don't update their rates very quickly but your credit card company does.
With the news of Brexit, the Pound has suddenly died, down 7.5% at the time of posting this. Simultaneously, the USD has picked up some 3.3%.
To give an example of how you can use this to your advantage,
I've been considering and reconsidering the purchase of a Xiaomi Yi action camera. It's been on sale at Banggood for US$63.39 for a few days. When you flick it over to GBP, it only asks for 43.19. At time of posting, using current rates (which you will pretty much get from 28 Degrees or Bankwest Zero),
US$ 63.39 = AU$ 86.47
GBP 43.19 = AU$ 78.93
That's a pretty tidy saving!
Edit: (Switching billing to Australian Dollars, it's AU$ 84.49)
Edit 2: Don't forget to check if your site is on Cash Rewards too - Banggood has 4.8% back
I don't know about Banggood, but changing the currency displayed on some websites only does just that — it changes the currency that is used to display the prices of the goods.
For eg a Cree torch could be displayed as $9.99 USD — changed to AUD — it will display at A$13.45 for instance. But this is only for show — you might not end up paying that exact amount!
When checking out at Paypal, it may default to USD payment, and you'll end up paying USD regardless and you'll be using Paypal's exchange rate.
Double check before you pay.