It depends on how you look at it. If you agree to purchase an item you enter into a 'binding contract' to purchase the item. If you don't complete the purchase your can be reported to eBay and be suspended. If you are a seller and refuse to send the item sold in the 'binding contract' for any reason (real or otherwise), you cannot be compelled to send the item by eBay. Technically, eBay can register a negative mark against the seller but as a buyer, I have never seen this displayed in a seller's ratings.
If you do not receive an item or one that if defective, you need to negotiate with the seller before you have any right of complaint to eBay. They will step in once a complaint can finally be filed and will usually get your money back for a lost item. For damaged or not as described items the seller may insist on the return of the item and (usually) may not agree to reimburse return shipping costs even though the seller is responsible fro the safe delivery of the item under eBay rules. Not so bad for local purchases but for international or heavier items this can prove expensive if the seller does not package it well to begin with.
You can complain to eBay as mentioned if you have an adverse experience with a seller. They will 'investigate' it and NOT inform you of the decision. You have the option to leave negative feedback for a seller if you have had a poor experience. If you have complained to eBay and they have 'resolved' the issue by reimbursing you etc they will remove any negative feedback you have left for that seller, so others may experience the same thing again from that seller. I purchased something from a UK seller recently, and in discussion with him about delivery of my item we chatted and he was surprised to have experienced exactly that because the other party had 100% positive feedback rating.
Any party can whine to eBay that it has received 'unfair' negative feedback and have it removed, recreating the 100% positive feedback rating. eBay is a US based company and there is a societal preoccupation with superlatives where nothing but a 100% rating will be adequate. eBay has a very vested interest in having sellers with excellent seller ratings - SALES, since it take a nice percentage in sales fees (plus shipping fees)
Finally you have the option to take up the issue with PayPal. It used to be that they would not deal with any eBay issue for obvious reasons as an eBay company but they are now separate so I'm uncertain about the new arrangement (eBay used to have the transaction fee from that too)
There are many very legitimate sellers on eBay and I have had some excellent experiences with them when things go well, and when things go wrong. As a frequent eBay user I have learned that a flexible and pragmatic approach is often required but there is a small(er) proportion of jerks on there so you take a risk when purchasing an item, which is frequently not a bargain. I use eBay when I cannot acquire an item somewhere else but for those of you who have the option to walk into your local model shop, or buy online from a model shop (almost all are helpful and reputable) that is likely to be much less at risk when an issue arises, and helpful advice available for things purchased and many besides. There are also other reputable vendors who have a love of modeling and railways that trade in these items.
This might be better on the eBay thread is Mod wants to move