To work out if the second hand solar panels are good value for money, besides their physical condition, look at the cost of dollars per watt generated, one disadvantage of buying used solar panels is that Government grants most times only apply to new purchases and not to used solar panels.
You can also try to find scrap solar cells, these are the cells broken during their manufacturing of solar panels and solar cell factories sell them very cheap, they are not very efficient but can be perfectly suitable for low needs projects, the only downside is that you will need more space because more solar panels will be needed, you will also need to solder them together or have someone with iron soldering skills do it for you.
You can find information on buying scrap solar cells contacting your local manufacturer directly, they can also be found on sale online.
Solar panel age: This will affect its productivity, thin film panels are specially affected by age, monocrystaline and polycrystalline used solar panels efficiency does not degrade so much with the time, solar panels have an average lifespan of 25 years, their wattage output decreases over time.
Well constructed and undamaged structure: For a solar panel to perform at maximum efficiency the seal should remain intact, otherwise moisture and dust will get inside, this can significantly affect its power output.
Solar panel internal circuitry: Check the solar panel for voltage fluctuation and voltage drops, this would indicate some sort of damage in the electronic circuitry which normally is extremely hard to repair without advanced electronic knowledge and specialist tools.
Test the voltage production with a multimeter while the panels are in the sun, a nominal solar panel power rating is always lower than real output in full sun.
Solar panel preceding use: Find out where the second hand solar panels were being used before, there might be issues if they were installed on a boat as corrosion caused by saline water might have affected the electronics internal circuitry.
Solar panels do not degrade in storage, solar cells that have been in storage will be more valuable than one that has been actively converting sunlight as the capability of hardly used stored solar panels to convert sunpower into electricity will remain higher.
Determine your home total wattage needs: You don’t want to overspend buying more panels than you need, do the calculations to find out what power output you need from the solar energy system before you try to find used solar panels, this way you will know exactly what overall wattage is needed.