The best time to sell your forklift is when you are not happy with its performance and the performance goes down considerably over a period of time, when the machine gets too old and it by all means will never fetch you the same performance for which you bought it.
One more instance when you think to sell a forklift is when most of the parts are so damaged that they cannot even be replaced, or if done, will make for a very hefty bill and it could damage your reserves very badly. It would be wiser to sell the machine so that costs are saved.
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Sell it for a substantial price, wrestle up some funds and get yourself a better forklift machine so that the work goes on well.
Another instance to sell a forklift is when a good alternative is available at a decent price and you need to get some cash to buy it. If it is better than yours in terms of performance and longevity, then it would be best to show yours the door and get that one in.
These are perhaps the times when you would feel like getting rid of your machine and getting something else in. These are great to use, but when their time period is all out then there is no use in keeping them just for the heck of it.
Get rid of them and get something better. It will only do good to your interests that will again become lively and productive and do what they are best at doing.