I think no matter how good or bad a used tool looks, there's always a risk/gamble, especially if there's no warranty left. I have repaired many of my own tools, that were bought new, and the warranty has run out. This is mainly because my stuff is looked after as much as a professional working environment will allow. So our tools usually last a long time, and with wear and tear, things will need maintaining or replacing at some stage, especially with age.
As much as I am capable of doing the repairs, I wouldn't buy tools that are in need of repair, even cheaply, unless it was a rare tool with a relatively easy repair.
My time is better spent working at my trade as opposed to fixing tools. That's not to say I'd never consider buying used, or ex demo or refurbished tools. It has to be the right tool, at the right price after taking it's age, amount of use and age into consideration.
I know people who have bought some lovely used tools at very low prices. Sometimes tools that although old and out of warranty, have been sitting around unused and still in an almost new like condition, for sensible money. These would be more my cup of tea, the price has to be right though, otherwise I buy new.
It doesn't matter how well made Festool stuff is, without knowing it's history potential problems and cost layout can be hidden, even brand new Festool fails sometimes. Take a Kapex 120 for example, the older model, If I were offered a used one, personally I wouldn't touch it with a barge pole, unless it was incredibly cheap, even then I'd be cautious.
I agree with the comment about the seller of the sander the OP is asking about, there has been no effort to tidy up the tool for sale? you could say the seller is being transparent, and saying "what you see is what you get" on the other hand, you could look at it and say, this guy doesn't care about his tools, it's been put away dirty, and almost certainly is dusty inside, and has the potential to fail?
When I replace tools, if the old tools that have been upgraded are not seeing use, they go into our storage unit, there's a lot of tools in there, sometimes kept for parts etc. Often I might give certain tools to a friend or colleague to help them out, if their tools have failed or been stolen etc. People often ask me what old tools I have sitting in there, as I'm known to own a lot of tools.
Every so often I might sell some of the more usable stuff off, if so it's cleaned up, and presented nicely and nearly always with a copy of the original receipt.
I get, what I think is good money for the tools, and they sell quickly but, they are priced according to their age and use, and what I consider their future usable life span to be.
I have even given some personal warranty with tools I've sold, as I'm confident they'll last. I've never sold anything near to half price though unless it was very new, and bought in error, even then half price would be my highest price.
I've also thrown some tools away that have sat a long time, out of date cordless drills etc, where the batteries have died, the tool wouldn't fetch much money in a sale. So if they're not wanted by anybody, or no use for spares, they get thrown away.
Just a few weeks ago, I threw out two real old Makita cordless drills, A Metabo cordless drill (POS) an old planer, and some old grinders.
So although I do sell and give away used tools, I seldom buy them. A little while ago I missed out on a used Mafell Erika 85e, this saw had only seen about an hours use and was up for £1000. By time I was told about it, and tracked down the seller, he'd sold it the day before.
He bought the saw, and hadn't put his tax away, the tax man sent him a demand, and he was selling anything he could to pay the bill.
I would have bought that saw in a heartbeat, and for more money, because to me, it was an exception. A great used tool, albeit minimum use, at a shockingly low price.
I am still choked thinking about it.
So, I think you have to judge each potential purchase on it's merits, whether buying brand new, refurbished, ex demo, or used. If the deal is right, go for it, if not, walk away.
After reading this thread, and taking into account discounts etc, and the photo of the sander. I wouldn't buy it, that's me though
