i purchased a used mac with no software can i install from app store

Only if the computer originally shipped with Mac OS X 10.7 or newer; for older Macs, an earlier OS, such as Snow Leopard, needs to be installed from a DVD first. Append the last three characters of that iMac's serial number to http://www.everymac.com/ultimate-mac-lookup/?search_keywords= and load the page to find out what its original and maximum OS version is.

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You can try Internet Recovery - hold down Command + Option + R until you see a spinning globe and wait to connect to Apple's servers - if the Mac came with Lion or later, it may work. Not sure though as we don't know which Apple ID that machine was associated with.