A question for you experienced fire service people here:

Would it be considered reasonable, or stupid, to buy a used 2-1/2 gallon water fire extinguisher off of eBay? I see tons of them listed there, much much cheaper than buying a new one. I assume these things have pressure seals that wear out inside them - can the extinguishers be rebuilt inexpensively if needed? What brands are good to look for or to avoid?

Thanks!