gbb, first, welcome to the forum! We have a lot of experienced eBayers here who are more than happy to give you good advice.
From what you state and the manner in which you state it, I presume you're relatively new to
ebay, at least for buying coins.
ebay offers a lot of pitfalls for new coin buyers. You have already been given some good advice: ensure the seller has a return policy (few offer refunds on S&H); check his/her feedback (more on this in a moment); nice clear images of both obverse and reverse and, if a peculiar or variety coin (say, an 1890CC "tail-bar" Morgan), detailed images of that part of the coin; clear description of the coin. Avoid auctions with vague descriptions (e.g., "You be the judge."; "See image.") or which don't say anything about the coin except it's date and denomination.
And as also mentioned, until your grading skills are honed for the type or series coins in which you are interested, buy only slabbed coins and those from ONLY ANACS, PCGS, or NGC. This cannot be emphasized enough: any other grading company's slabs (examples: NNC, NTC, ACG, CCGS, PGS, SGS, and a couple dozen more) are at best inaccurate, at worst are fraudulent. Stay away from high-graded and/or high value raw coins, especially the rarer coins such as 1889CC Morgans; they are most likely problem coins (cleaned, damaged, or otherwise ungradable). If they were so valuable, they would have been already graded and encapsulated to protect their value.
On feedback, don't go by the feedback percentage; a 99.7% positive score on 4000 transactions is very bad indeed. READ the negative, neutral, and "mutually withdrawn feedback" which can be obtained without a lot of scrolling at ToolHaus:
http://www.toolhaus.org/cgi-bin/negs . Also, usually the seller's responses to negative feedback will indicate his/her character. In many instances, negative comments can be read between the lines in positive feedback.
Avoid "estate sales" like the plague; similarly, watch out for coin sellers who "don't know anything about coins". Avoid "private" auctions in which the seller's feedback or the bidder's ID is hidden; the seller has something to hide. Avoid sales from foreign dealers until you've built up some experience and can differentiate between legitimate and fraudulent sellers.
For me, if I see a coin I want, either slabbed or raw, and it is likely to get into several zeros behind the dollar sign and the seller's information (above) looks good, but I don't know him, I send him an email asking specific questions about the coin, e.g. for a raw coin, "I like this coin, but I can't tell for certain if it's been cleaned or not. If I win it, I plan to submit it to PCGS to be graded. If PCGS will not grade it due to problems, will you accept the coin in return?". This question, if answered honestly by the seller, has saved me a lot of grief and a LOT of money.
All of us eBayers (I've got seven years behind me) have been burned at one time or another. It's part of the learning process. So, don't bid on high value unauthenticated coins unless you have money to lose. And no matter how good the images might be, it is still almost impossible to adequately evaluate a coin without having it in hand.
I (along with many on this forum) could write several books on
ebay buying and selling, but this will have to do for now. In summary, education and experience is the key to
ebay success.