I've used used tyres but after thinking back it was just waste of money and when I want to go somewhere, I have to think twice, as tyres are not good.
The used tyres you find in shop mostly are not that good ones, unless you are lucky. Most tyres are worn out un-evenly only a side or so. The best thing would be, if you can't afford to buy 4 brand new tyres, otherwise buy at least two tyres and after few months or few weeks, go for another two tires.
Used tires are just a waste of money if you are planning to use the vehicle a long time.