"It's nothing about being inexperienced, just a matter of picking the right battles. In my cases I have confronted the sellers, and all say that I have broken them myself, so it's my word against theirs, even if I would have involved ebay or not."
from where i sit it seems you are not going about things the right way. it is always your word against theirs no matter what. but paypal disputes get judged. usually a guilty man won't put his side in these disputes. thats why you get the money back if they don't try to resolve the issue. if it is faulty and you know it came faulty. it is the seller who thought "it's broke. i'll ebay it. he he!" or the postal service. you need a refund. and maybe if the seller is 100% he will try to resolve it with the postal service for you as he should get a receipt for postage like i always do. it costs nothing. i have had about 20 faulty items in my time and of all the disputes. paypal has found in my favour. the money comes out of his account not paypal. and so doing disputes is the only way of keeping the idiots honest. because they loose out + postage...
don't leave feedback till you are happy.