It's not like there isn't plenty of information on the web about how to pack it right. GoooOOOooogle "packing a turntable". Couple good ones ...
http://www.ebay.com/gds/How-to-Pack-a-Turntable-for-Shipment-/10000000002161041/g.html
http://www.oaktreevintage.com/turntable_packing_for_safe_transit.htm
YouTube has plenty of titles for those who need pictures ...
I like blue construction foam. Takes maybe half a sheet to do it right.
Cut a piece for the bottom of a box that's a couple inches larger than the max footprint dimension of the table.
Cut out another piece for the platter. That goes in next. And platter isn't just the mat - we're talking the entire spinning portion here. Most are gravity fit to the spindle - could be a bit hard to break loose if it hasn't been off for a while.
Another piece between that and the table body. Good place to do cutouts for weights and such that you don't want bouncing around inside.
DO use any "travel" hold downs provided by the mfg to secure the plinth. DON'T trust the arm hold down to stay in place. I always add twist ties in strategic locations, and make them easy to spot. Objective is NOTHING loose inside.
I prefer to leave the dustcover on. A crossed piece of blueboard inside will keep it from crushing. Think beer crate - a couple notches and slide the pieces together. Tape some soft cloth or tissue to the end that touches the cover to prevent scratching, and make sure the foam doesn't sit on any of the sensitive bits.
IMPORTANT - Don't use tape to hold the cover down. Stuff can be impossible to get off at the other end. Couple layers of plastic wrap works nicely. Also put some pieces of cardboard between the edges of the cover and the plinth so it sits level. Most have little nubs at the corners that will break off otherwise, and you also want to make sure you're not stressing the hinges.
- Drop the table in and cut pieces of foam for the sides. Make sure it's a tight fit. Multiple pieces of foam and bubble wrap should do the trick. You can do cutouts in the panels for the cords. Tie those and a quick wrap of bubbles to keep them from rubbing against the table body is a good thing.
One more piece of foam on top, and you're pretty much done. Fold the top of the box over tight and tape the hell out of it. Don't forget to include manuals and notes for reassembly, including travel locks and such. Also include a copy of the bill of sale with your full return address. Labels get ripped off boxes, and this way, the shipper knows what to do to get it home.
If you get in on the sale early enough, contact the seller presale with your concerns, but that's not always possible. I ALWAYS email the seller (use eBay's internal system for a paper trail) with my concerns and suggestions immediately after purchasing as well, either as a heads up or a reminder. Helps with any claims that may need to be followed up if it all goes horribly wrong.
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Granted, not everyone is going to want to go through all that and may cut corners on packing anyway. Letting them know what you expect WILL however convince them to do better than just slapping a label on the dustcover and dropping it in the corner pickup box. Every little bit helps.
Not that it's all on the seller. When you get the table (or anything used) keep in mind it IS used and liable to show some wear and tear. Cosmetics can be dealt with, polishing the cover, a bit of touchup on the plinth, new cables, etc. A bit of TLC on the internals is also to be expected. Short version, don't expect plug 'n play out of the box, but be prepared to be pleasantly surprised if it turns out that way ...
PS ... this IS the short version ... ;-}
it's sometimes easier to contact ebay directly and speak with them (google for a #) it's easier when you have ebay on your side.