VEG is right ,
its a nightmare especialy getting a saw off ebay , you have to be careful that the "big name " saw is genuine and not a cheese copy from far far away land, be wary of secondhand stuff too , how worn is it ? does the chain brake work properly ? is the cylinder bore scored ( hard to tell unless stripped down ) not stolen.... and a hundred and one other things.
If it is just for very occasional use hire one, otherwise,
The cheapest way , in the long run is to buy from a real physical dealer.
New or secondhand tell them what you want it for and your budget, you may pick up a good secondhand one failing that if it has to be new console your self that it will have a warranty ,( even a second hand one will have a short one) you should be able to get spares and service and most importantly advice.
Our local dealer wont sell you one until he has demonstrated it ,made sure you can use it and you have the proper safety gear as he doesn't have arms and legs in stock !
When you get one look after it, use the correct mix , keep the chain sharp and when you finish using it run it dry of fuel if not to be used for a while clean out the crud that has accumulated ,give it a wipe over and if possible get a proper case for it ( not an airtight one).
My stihl saw is 26 years old ,ok its had a new carb, but starts 1st or 2nd time . the way I look at it is the purchase price divided by years owned..
What ever you do , do it safely