This guide is aimed at people buying and selling vintage electronic music gear on eBay, although a lot of it also applies to ANY selling and buying on eBay.
It's based on my experiences of buying and selling this kind of gear over the last couple of years. Others may disagree.
Make sure that you research everything before you bid. There are plenty of information sites. Vintagesynth, Synthmuseum, Harmony Central, Sonic State etc. Don't just use one. Use Google.
Use the eBay advanced search function, and select "completed listings", to see what sort of prices similar items went for.
Check into shipping costs yourself. It may be cheaper to arrange your own shipping. Contact the seller to see if this is acceptable. If the shipping is too high, DON'T BID. Another one will come along..
Make sure that anything you send is safely packaged. For a synth, this means bubble-wrap AND cardboard, at the least.
I've had a synth sent to me in a plastic bag. The end was smashed. The courier denied responsibility. Result: sold as spares, and a loss.. Don't do it.
3. POWER SUPPLIES.
Check that an item has the correct power supply for YOUR area. If you are selling an item, make sure that you state the power supply specs. If it's UK, 3-pin, 230vAC, then SAY SO.
I had an Italian buy an ALESIS drum machine from me. It was UK spec, 3-pin, 230v - 9vAC adapter. So he went and bought a cheap Italian-spec, 9vDC adapter, plugged it in, and fried it. The key point here is that he didn't check. He then tried to blame me for this, and demanded his money back. Fortunately, he'd e-mailed me about what he'd done, before he tried this. I still got negatve feedback, though..
IMPORTANT: ALESIS gear needs 9VAC power supply.
Read these carefully. If you are not sure about anything, contact the seller. If they don't respond, don't bid.
If you are selling, BE HONEST. A buyer would rather know about a fault up-front, than hassle about it later. Chances are, you will get MORE people interested, anyway.
Also,the more info you put in the listing, the less questions you will get.
This is the key to eBay. A seller should be prepared to answer ANY questions. A buyer should ask any questions before bidding.
If you get no response to your questions, or encounter a bad attitute, don't bid.
Don't bid on anything where the only method of payment is Western Union, or anything like that. Be very wary about any seller who only accepts Bank Transfer, as you will have to give them your bank details.
PayPal is actually the best option, due to it's integration with eBay, currency conversion, and speed. Having said that, PayPal eCheques take ages and are pointless. Avoid.
Expect faults and glitches with all old gear.
Don't pay too much for something that's not working.
You may end up with an expensive paperweight!
8. REPUTATION / FEEDBACK.
Check the seller's feedback. Not just the numbers. READ the comments. Especially any negatives.
You will get a feel for the sort of person from this.
If in doubt, don't bid.
On the other hand, a new seller will have low feedback, so ask a couple of questions, and see what impression you get of the seller.
As a seller, you really don't need disputes and negative feedback. So be upfront and honest. Your feedback rating matters.