Before you make online purchase 1: Check Your Browser:
Use latest web browser. Use latest version of google chrome or Firfox, both are good and safe. You can install on your mobile too, Most Android phone are pre loaded with chrome browser. Samsung mobile phone have its samsung web browser. Do not use rooted device are jail breaked etc as its means these are already scurity disabled and not secure. keep your device upto date, manufacture are giving update regularly.
2: Website security:
Make sure your device time is correct. When you browse any secure website its address starts https and none secure address starts http. there is "s" in secure web address. Whene you browse none secure web page, latest browser always warn you about its none secure, SSL cirtificate is not valid. Its also warn you when your internet connection is not secure.
3: Business and Seller:
Always check online seller location as some sellers business do international trading. It will take long time to deliver your item from internationl seller.
Check business is new and how long it is in trading. as most new business and seller could be fake and rip you.
Check Business history and reviews on social media site trustpilot etc. it will give you clu about seller is good or no.
Check Business contact details. It should be easy to reach your seller or contact him. Website must have contact details. make sure number are normal like starts from 01, 02, 03. contact email. if number is 0844, 0843 or 09 it will cost you.
4: Payment method:
Before making payment, read seller payment information. Always use trusted payment method like Paypal etc as you can get help from them if something happend wrong (for new seller must use paypal). if business is old and you already know about it, you can use any payment method. but Never do bank transfer, westerunion, moneygram or similar method. Also beware from call order etc, Never pay over phone to strange business, Charity or too good deals.