You've been given some good advice, but as someone who has sold and bought a bunch of stuff online over the years, I'll add my usual 2 cents! Here's a few things I consider absolutes:
1) Check the seller feedback. If it's not high 90's of %, then move on.
2) Check the seller history. If this is their first listing/sale, then move on.
3) NEVER, EVER, EVER agree to pay via something like Western Union/Moneygram. Always use something like Paypal or a Credit Card.
4) If the seller is a company, are they a certified shop or a dealer of respectable brands? There are plenty of good companies who sell avionics on the internet (I just bought a bose headset from Pacific Coast on eBay, because I know without any doubt they are a good outfit).
5) Is it shipping from the US/Canada/WesternEurope? Avoid avionics shipped from Asia...they are often times stolen.
6) If it's too good to be true, it probably is. If someone is selling an "almost new" 430 with no tray/backplate/connector/antenna for $1000, then there is a good chance it's also hot.
7) Ask for the serial number, then call the mfgr to check and see if it's good. Many of them keep a list of reported stolen units. Also, if you can find any sort of history from the seller (like a recent upgrade), that will shed some insight into the condition of the unit.
Buying from individuals can be just fine, but I've also seen some absolute horror stories. We often times have received goods customers bought on eBay, only to have to tell them they are absolute garbage. That said, sometimes you can also end up with a great deal as well.
If you follow the advice given, then you can end up with some OK deals...but realize that the good deals end up selling for exactly market price. If they go for much less than normal market price, then there is likely "something" weird with them.
Just my 2 cents as usual!
Cheers,
Stein
