other than ensuring it's rated for well over your aquarium volume?
I would make sure that the model has replacement pumps still available.
Skimmer bodies rarely fail.
Pumps do take a lot of wear and tear...properly maintained, a good pump will last for a very long time but when you buys used, you roll the dice a bit on the condition of the pump.
If it does fail, no biggie as long as there are replacements easily acquired.
There are definitely some lower end brands with excellent reputations as mentioned above that won't break the bank new....even these will be cheaper used though. ;-)
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