Purchasing used scientific equipment can be difficult.  Does it work? Is it clean? Has it been decontaminated? All of these are very important questions.

Most sellers aren't quite sure what they have, if it works, or the history of the instrument.  It's recommended to purchase used equipment through a seller who has an excellent feedback rating or has an established equipment business outside of their Ebay transactions.

Instruments with no or few working parts are easier to purchase.  As the complexity of the instrument starts to increase, you should as a buyer start to communicate with the seller regarding the functionality of the equipment.  Frequently when equipment has gone through a few auctions or has been disposed of as a surplus asset, the critical components, which are also the most expensive parts, can be missing.  Asking the buyer to plug in and try to power up the instrument is acceptable.  However do keep in mind that most sales are 'as is, where is' and the responsibility for testing and refurbishing is yours.  Be prepared to do some work on your end after the instrument arrives.

Those with technical and engineering backgrounds have great success purchasing parts or non-functional units for parts.  If you do not fall into this category, before you purchase do some basic homework.  1)  What is the instrument? Do you understand what it does.  2) How will this instrument work for you? How does it fit into your project or application.  3) Are there still replacement parts available?  4)  Do you know someone who can fix the instrument for you or install any replacement parts you might have to purchase? 

Remember, there really is no such thing as a free lunch, so be prepared to put in a little time and in some cases a little money to get the instrument up to 100% functionality.

Keep in mind that most shipping services will not pay for damage on a used item, or they make it so difficult to recoup any costs that it's not worth the time.  Ask your seller to package all glass properly and the instrument appropriately to try and insure that the unit will not be damaged during shipping.

Finally, as a precaution decontaminate the instrument upon arrival following your protocols.  Better be safe than sorry.

Used instrumentation provides a wonderful value to any project or any corporation, by employing some common sense and knowing what you are getting into before your purchase should help you have a smooth and successful transaction.