The key, of course, as in any acquisition, is to know what an item is, and what it should cost.

I've purchased quite a bit of used test equipment over the decades, and have a few words of advice, in part relevant to used equipment purchases in general, and in part, specific to eBay.

First of all, be certain that you know exactly what the equipment you're buying is. Obtain the manuals, both service and user manuals, often available in CD form for about $3-$20US, depending on the type of equipment. All this will be made much easier if you limit your choices to TEKTRONIX or HP instruments. Hitachi and Iwatsu both make very good instruments, but they are not so common, hence emergency repairs may be much more difficult to effect, as parts availability is a function of how many such instruments were sold.

Secondly, always wait until you find an instrument that is relatively complete. Generally, I don't buy a logic analyzer or oscilloscope unless it is accompanied by all the equipment with which a new one was shipped in its day. This is not always possible, but you'll find it's very inconvenient, to buy an oscilloscope, only to find that the appropriate probes are unavailable.

If you buy an oscilloscope probe, be certain that it is complete and intact. There are lots of probes being sold, which lack the hook-clips because they've been lost or damaged. An oscilloscope is difficult to use without them, as these are required to attach the probe to an IC pin or test point. Be certain, also that there is an adequate ground strap with the probe as well. The hook clip and the ground strap are often missing, and the probe is useless without them. Be certain that you know what the rated bandwidth of the probe is. I don't recommend using a 50 MHz probe with a 100 MHz 'scope.

If you're looking for an "experienced" 'scope, it will undoubtedly be an analog type, which I like very much, BTW. If you buy a TEKTRONIX or HP instrument, you'll have little reason to curse it, by my experience, though you should have a clear assertion from the seller that it is fully functional and complete, as specified in the advertisment, and that the seller will refund your purchase price, and pay for the instrument's return if it arrives in any state other than fully functional. If you like the TEKTRONIX 465 or 475 series of 'scopes, keep in mind that they're often equipped with a DM44 multimeter. These are very handy, but they do require a few additional accessories, notably the temperature probe. These are really difficult to obtain if you need one. The DM44 is capable of making quite precise time and frequency measurements, as well as measuring voltages and resistances. I've never used one to make current measurements, so I don't know whether they do that.

Your 'scope should have two channels, both internal and external triggering capability, trigger holdoff, and a delayed timebase. If you use a TEKTRONIX 465 (100 MHz, shipped with 6105 probe), 475 (200 MHz, 2mv sensitivity, shipped with 6106 probe), 475A (250 MHz, shipped with 6106 probe) or '485 'scope (kind of fussy, but 350 MHz bandwidth), you'll find that parts are no longer easily replaceable. Earlier (with vaccuum tubes) instruments are so heavy as to be prohibitive to ship and the parts are virtually unavailable. Instruments of later vintage than the '80's are somewhat less convenient to use, as they trend then was toward computer-controlled test instruments and many compromises in front-panel useability sufferred as a result. HP led the charge in this sense, and, BTW, TEKTRONIX 'scopes triggered considerably better than HP's of that period.

If possible, ask the quesitons via eBay's email message service, which is the default, and which will provide a record of any communications. That way, if anything goes awry, you'll have little trouble getting a resolution.

I've had really good luck overall, having bought over 100 items, some of very substantial cost, and only had to have eBay and PayPal become involved on two occasions.

BTW, PayPal is definitely the way to pay for things. They allow you to use a credit card, if you wish, yet don't require you to put your personal banking information at the disposal of each and every seller with whom you deal. The threat of requesting them to resolve a difficult situation is often sufficient to produce a prompt resolution from the seller.

Your best course will be to obtain an instrument with which you're already familiar. That way you'll know right away what equipment you need and whether the instrument works to your satisfaction.

Above all, however, do not buy an instrument on which you have no right of return. Sellers who won't refund your purchase price if the instrument is unsatisfactory, at least to the extent that they reflect the details in the description and in the answers to the questions you've asked, is not worthy of your trust.

RE