Hello there. I have been buying diecast as well as selling on eBay for a while now, and thought perhaps it might be good to pass on a few tips I've learned. This is not a fool proof, must do all these things list, just a few things that seem to work well for me, so hope it is of some use to you.
Finding Your Diecast.
This might sound obvious, as you already know what you're looking for, but it can sometimes be easy to miss the model you seek. Diecast is one of the largest catagories on ebay, with regularly over 100,000 models listed at any one time. If you are looking for a certain make, such as Dinky or Corgi, it might be easy to assume you will see them all in the correct catagory, but often they can crop up pretty much anywhere, as many sellers seriously do just have a box of old toys from the loft, and sell them individually, but forget to change catagory. This also goes for sub-catagories. I always set my search as wide as possible, and generally find a good selection out of place. I tend to buy 'commercial' vehicles, and you'd be amazed how many turn up in 'cars-unboxed' or similar. Also, don't forget the 'job-lot' or 'other' listings. I often pick up some great finds just by carefully checking the pictures if there is a pile.
Is It Really A Bargain.
Isn't the best part of searching finding that item that you just must have, it's sitting at a price thats just to cheap, and it's worth at least three times as much. I know, I'm there at least four times a week as well, but as the old saying goes, 'all that glitters'. Before you hit the bid button, have a good hard look at the listing. I'm not saying that every cheap deal is a ripoff, but if it is unreasonably low in price, others may have avoided it for a reason, so check the terms and conditions carefully first. Some sellers have been known to set a low price and up the postage to compensate. It is one of the oldest tricks, but I still get caught on occasion. Do they have a long long set of terms and conditions, read them carefully. My pet hate is seeing a Paypal listing, then finding in the terms that they only accept them from abroad. Paypal is sometimes an expensive pain I know for a seller, but it really is a buyers best guarentee, and I rarely buy any other way. Lastly, look carefully at the picture. Some sellers may not be aware that the model has bits missing, or have not mentioned that the fenders are crumpled or the chassis is cracked. It may not be intentional deception, but it's annoying having to send a model back or start a dispute, when it could have been avoided.
Bidding Your Best
This is the tricky bit, and also the most common sense I guess. Diecast listings seem to either get no bids then a flurry at the end, or a steady stream of folks interested. Unless your model is particularly rare or special to you, always remember that there are many thousands of identical ones out there, and that another will come along. Set your price and stick to it. If possible, leave it as late as you can before placing your bid, or use a sniper engine to hide your interest. Sniper engines are very useful if you are a regular buyer of similar items. You may not realize it, but other eBayers are able to see what you are placing 'proxy' bids on, and if they know you collect the same as they do, they will simply let you do all the hard work of finding the gems, and then follow your lead. A sniper engine will stop this, it bids at the last second for you, hiding your interest, it's only downside is that as you are bidding last, you will pay slightly more as you have to top someone elses bid. I personally find Auctionsniper a great tool, and use it most of the time.
Making The Most Of Winning.
If you have won your item, have a quick look at what else the seller has to offer. Most Diecast sellers usually have more than one item up, and most will usually offer combined postage. It's a great way further reducing the price, and who knows, if you end up with surplus models you could have a go at selling as well, and your model might eventually cost you nothing.
Hope this has been of some benefit. Diecast collecting on eBay is a great and safe hobby, generally a lot cheaper than the toy fairs, and great fun. Out of the hundreds of models I've bought, I've never had a problem, and it does keep the Postman busy ...