No it is not okay to buy a second hand pair! Clothing and gadgets are two things that lose their utilities after a certain number of uses, so don't buy a second hand pair - just buy another type of good new shoes!
I want a pair of Nike shoes, but they're too expensive. Is it OK to buy a second-hand pair?
Better you buy the new shoes only as the second hand may not be worthy. Might be expensive but it is worth spending on Nike collection. You can have a look at updated Nike offers here.