At first let me doubt every people buying a reel to reel recorder know what they are doing really! Keep in mind i.e. the Revox B77 has been produced in 36 different models with the same outside look (see this B77 models reference table)
Recently I was stun by an ebay transaction. A B77 recorder with no pics and few informations, super low speed model (absolutely useless today) sold at very high no sense price. I guess the buyer will have a bad surprise! So:

• First step read carefully these full of news web pages. They will help you to choose the right tape recorder for your needs avoiding waste time and money for something useless.
• Second please don't dream. If you think you will find an used recorder "as new" or "NOS" perfectly working, you are dreaming. Depending on its condition every used recorder needs at least a full cleaning and alignment till to expensive repair to meets again the manufacturer specification (see the servicing page too). A tough apart about NOS devices. I resume my opinion with a question: would you buy a new 25 years old car laid in the manufacturer store paying it double compared to a same aged, less used and properly serviced one? Give yourself the answer.
• Third watch carefully the seller description. Often there is some confusion about the terms "as new", "looks new" and "new". Recently I saw a Revox sold as "NOS" because it had the original packaging only (!). Really the recorder was not NOS but fully serviced.
• Fourth be careful. Lately there is some clamor about reel to reel recorders and accessories (especially Revox here in Europe). This means you will find on the market used recorders in very different conditions and some astute seller. Keep in mind not everything called "Revox" means it is REALLY a Revox and not everything is really Revox has to be necessarily bought! Be alert when a seller looks like he doesn't know what he has in its hands to sell. Sometimes is a great way to make you believe you are outsmarting and making a good business. It is exactly the opposite.
• Fifth: ask, ask, ask! If you can't see the recorder before paying and buying it don't scare to ask pics, news, serial number, and so on. If the answer is: "sorry I don't have the digital camera" or "I just packed the device and I can't read the number and take a pic" or "I don't have the device here because it is not mine, it is of a friend of my (why the friend doesn't sell the device by itself ??) " or "believe me it is PERFECT condition (but no pic)" etc. don't persist in and wait the next chance unless it is a very rare or special item.
• Last but not least "watch the heads". Heads are the most expensive part of a tape recorder and their remaining life is very important. Changing an heads block can cost as buying another used recorder!! Usually you can easily access and check the heads. The following pics show a brand new head (A), an used but at 80% still very good condition head (B), a totally worn head in regular way (C) and a totally worn one in wrong way due to zenith angle problem (D). Some AKAI and TEAC TASCAM hard permalloy and/or glass ferrite heads usually don't show this sign of wear on the surface but a kind of black blots or spots. This equally means the are hardly used and need to be changed.

A                                                                B                                                                C                                                             D