J-Hulk,
The most important thing to do is, get a list of kits you are looking for and find prices that you are willing to pay (say from your normal online dealer or a favorite hobby shop) Example: I am looking for kit xx and at my dealer it's xx dollars.
Now, congrats, you found that exact same kit on eBay with a starting bid of, let's say $5.00 Always look at the shipping charge from the seller. Most start their kits or whatever at a very low price but, kill you with the shipping charge. Since I run a Shipping-Receiving Dept. I know roughly how much the cost would actually be. I would never pay anything over $6.50 ground shipping (normal size kit) in the US or over $17.00 overseas $17.00 being for airmail, not surface shipping(remember, insurance to protect it is always extra and the larger the kit, the more expensive the shipping). Back to the bid, so the kit is at $5.00 and the shipping charge is at say $4.00. How much more will it take to get just below the best deal you have seen this kit go for? I don't mean a one time sale but, normal pricing.
Always make your first bid, your last. I mean, bid your maximum the first time. You have to tell yourself, "I
will not pay anymore for this item" Why should you? You can get it at the other place for roughly the same price. This also helps with you having to rebid or a bidding war at the end. People love to "SNIPE" or outbid you in the final few seconds of an auction.
I always check the Tamiya, Dragon and Academy kits. There are some really great deals to be had, if you don't overbid. I have seen people sell entire Tamiya lots, dirt cheap, because they were moving on to a different form of modeling or just didn't have the time anymore.
Remember, when bidding, you have to say, you'll get 'em next time. I had the same happen when I was looking for my 1/16 Tiger 1. Finally, I caught it at the right time and with a seller I was familiar with. I won several auctions from him before and he sent it to me with almost no shipping charge from overseas.
One final word and probably the most important: always,
ALWAYS read the description. If it sounds shady, it more than likely is. If you have any doubts, email the seller. If he-she fails to respond or gives unclear information...move on. A good way to tell if a seller is on the up and up, is to read feedback from people who won auction items from them. Too many negative feedbacks stick out like a sore thumb!