Understand that buying ANYTHING used, is somewhat of a gamble. Buying it sight unseen (over the internet) is even more of a gamble.
Generally I tell people to use their best discernment and that you need to be able to trust the person you are purchasing it from. Will they take it back if it's not exactly as described (ie legal, fresh, etc.)? No one is looking for a "guarantee" simply a fair deal...and there are many out there.
If you can afford, I'd recommend taking your used engine purchase directly to the engine builder of your choice (whether that's one in your area, or one that you've met/trust online.)
As a builder, I regularly have customers ship/bring used engines in to have them gone through.
I don't mind it a bit. If it helps a racer better afford his racing, then I'm all for it.
I don't particularly care what sticker is on the engine, just that the machinework was done correctly previous to my getting it on my bench. Buying an engine that came from a major/reputable builder, is generally a good choice (as long as the decal on the blower housing is the one that came on the engine originally. If we've got to re-do everything (machinework, updates, cam, etc,) then it's not going to be such a great deal anymore.
You will find that most engine builders also offer used engines for sale (fresh.) It's hard to beat buying directly from a builder, since you know that their reputation stands behind what they are selling, and that the engine will come with customer service after the sale. That assistance offers peace of mind, especially to the new and low budget racer.
Like others have said -- there are some great deals to be had, and there are some bummers out there.
Ask lots of questions and use your best judgment.
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Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cut
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www.CarlsonMotorsports.com
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30 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com