My best advice is know what you are buying. Ask the seller questions before you buy. Find out what the unit cost new, how old is it, is the person selling it the orginal owner. Compare the saw HP & CC to a new model in the same class. Saws come with various choices of bar sizes. If you are doing tree branch trimming a 14 -16 inch is perfect. If you are cutting firewood for you home then 18-20 is a good choice and 24 inch for bigger more powerful saws that can handle this size bar. Consider not only the cost of the saw but shipping cost. Some parts like the bar, chain or air filter may need to be replaced when you actually receive the item. Parts could be missing, cracks in the housing or metal covers from wear and tear. Some people take very good care of their equipment other do not. If the item is sold "AS IS", no returns and not working it could be junk. These product are 2 cycle which means you must mix formulated 2 cycle oil with gas to obtain the correct mix. If this is not done the saw motor could be ruined in a few minutes of operation. Many used saws show up at pawn shops and these people for the most part know nothing about chain saws. The units are sold "AS IS" no returns, many say they do not run or will not start. The problem could be a minor issue or very serious. A saw dealer is going to charge you $60.00 + per hr labor plus cost of parts to fix the unit. If the motor is trashed you could spend $250 - $300 for repairs. Many saws sit around with gas in them and foul the carburator, then it will not start, idle or run. This is often the most common problem and it can be fixed for about $50 - $75 at a dealer shop. If you are good at fixing things a carburator kit is less than $10.00 and you can do the repair yourself. There is nothing you can spray on or into the carburator to correct the problem. The part must be removed from the saw, torn down, cleaned and a rebuild kit applied. Then reinstalled, adding new fuel lines and gas strainer, making LO & HI + idle adjustments during startup. Start at 1 turn out from dead end for LO & HI and that will get you very close to perfect. Twick both the adjustment slightly for best performance at sea level and higher elvations. Idle can be reset up higher by turning the screw in, lower by turning out. If the chain is moving on the bar in idle mode you must back off the idle adjustment (screw it out). If the saw hesitates when you pull the trigger then make a slight adjustment to the LO side screw (screw it out 1/8). The LO side and Idle adjustment are somewhat connected. When you change one it effects the other. Be paient, make small movemnets (1/8) in the adjustemnt screws until you get it perfect. The final step is the high side adjustment. This effects the RPM's of the saw when you are cutting. There are only three saws to buy new or used and they are Stihl, Husky and Echo. Many of these products can be purchased new online for a great price, free shipping, no tax and full factory warranty. Do you homework, be a smart shopper and don't loan your chain saw to anyone.
Good Luck, Patrick Swafford in AZ.