Based on my 30 years of collecting (and selling) Golden Age comics here are my top tips for buying them on eBay

1. Buy CGC comics if you're viewing the comic as an investment.
2. If the comic isn't certified, "certify" the seller. Carefully check his feedback and the feedback he's left for others.
3. No matter what, make sure there's a return policy that's as generous as possible. You just never know what can happen enroute to the comic getting to you, or the seller may have missed something such as a cracked or badly scratched CGC case.
4. Ask questions BEFORE you bid. You should do this not only to make sure you undesrtand completely what you're buying but also to get a feel for what kind of seller you're dealing with. Is he responsive, or is the type of seller who views answering emails as a bother.
5. Be sure you understand and are comfortable with the shipping charges. They range all over the place for Golden Age comics. Sure they require extra protection, but if the seller is charging $15.00 or more I would question his ethics. He may view S+H as a way to pad his profit and reduce his eBay fees. This is not to say there aren't honest dealers charging as much as $20.00. This can be justified if the seller has the item professionally packaged and ships it priority mail or better.
   
6. Know the value of what you're buying. That means to check the Overstreet Price Guide but also check Completed Listings on eBay, as those listings are more current than Overstreet and reflect  real world prices.
7. Check the Heritage web site. Heritage is one of the great auction houses for coins but they hold a lot of auctions for comics also (including many on eBay). Their database (just google Heritage comics) provides data on every comic the've sold and it's likely they've sold the one you're thinking of buying or one like it.

For more information on checking out sellers, or becoming a seller yourself see eBay PowerSeller Secrets, How eBay Really Works, or eBay PowerSeller Million Dollar Ideas. And enjoy your hobby!