Although I am a military trained electronics technician and have 37 years professional experience with Music amps and equipment and am thus qualified to work on amps, ...I am not set up to do so on a reasonable scale and have no intention to do so in the foreseeable future.  I do not seek out amps to buy and sell.  I have and sell amps only incidental to them finding their way to my shop;  And I do not bench test them, so my knowledge of an amp I might have is limited to external observations and sometimes my use of them.
When I sell amps I've acquired, I sell them AS-IS with no warranty nor guarantee of their condition, and no refund.  I might have a working knowledge of some specific amps I might have for sale from time to time, and am glad to provide what info I know about them.  When someone might desire to buy an old and / or used amp from a shop not set up to test and repair them, then I highly suggest they anticipate that the amp will need repairing or reworking to suit their desires & needs when they receive it, AS SAID FURTHER BELOW:

Whenever it is possible for a person to do so, ...I recommend purchasing a used / old amp from a reputable source, within reasonable driving distance and who is able to repair it (and thus advise you about the amp and it's condition they should have tested).  But of course you might desire an amp that such a place doesn't have;  So you are left to search elsewhere.  Elsewhere should again be someone who is qualified and set up to work on amps and again thus able to advise you about an amp they should have tested and be knowledgeable about.  Otherwise you might take the following considerations into account.
Used amps, in particular old ones, are notorious for acquiring problems at any time, particularly in shipping.  They can work fine when shipped and arrive with problems caused in shipping, or develop a problem when received and plugged in / played.  Many if not most knowledgeable people purchasing used amps, particularly old amps, anticipate that repairs will be needed after shipping.  Anticipating that in your amp-buying budget is a win-win situation:  You will either have that expense covered should it arise, OR you will have extra money / gravy on yer biscuits.

OTHERWISE, a used or old vintage amp should be purchased from an entity set up for and specializing in them, AND offering a guarantee / warranty.  Of course finding someone who specializes in old amps and has what you desire can be quite a challenge, and acquiring what you desire while aniticipating it needing repair or reworking is the best way to cover your 6:00 o'clock.
There are generally 2 perspectives for purchasing old amps:  (1)  Purchasing as an all-original, vintage, collectors item or desire for an all-original amp;  In which case such an amp should be completely gone through by a qualified / experienced amp technician AFTER receiving it, to rework it specific to your reasons for acquiring it.  And (2)  Purchasing as a performing / playing amp;  In which case most knowledgeable techs and professionals advise that the amp be reworked with modern LOW-FI (or hi-fi if you desire) componants that meet the specs of EITHER the amp's original specs OR to newer, better or different specs you might desire upon consult with a qualified vintage amp technician.  Point to point reworking of an old amp with modern and more reliable componants can be very rewarding, ...and is my personal preference for all amps other than those specifically purchased as an all-original specs collectors item.

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My Electronics Links page has a number of excellent resources for amplifier information and data;  Particularly Amp Tone's website. which specializes in resources for understanding and getting whatever amp tone a person desires.

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