want to buy ram for my imac 27inch late 2013

You cannot buy RAM at an Apple Retail Store. Instead you can only buy it online or at a specific shop.

There are different ways to buy RAM, including the Apple Online Store. However, I recommend you to buy RAM from OWC or Crucial because they sell the same RAM as Apple and it is much cheaper.

If you buy memory from another source, make sure it meets the iMac specifications > http://support.apple.com/kb/HT5908

I have not read about any problem with OWC memory in Late 2013 iMacs. I remember reading problems with Crucial RAM, but not OWC.

If you are not sure, you can purchase RAM at the Apple Online Store, but note that it is expensive > http://store.apple.com/us/memorymodel/ME_IMAC_F13_27

I have been running 16 GB of Crucial RAM since January -- no issues.

Remember that just like this forum, most reviews are posted by people who are unhappy, not by the many many who have products that work and are busy using them.