I understand a couple of disadvantages here would be the general inefficiency of the blockchain, and zero protection in cases of fraud. Do you feel the benefits of the blockchain would outweighs the negatives here?
The general (electrical) inefficiency is a specific characteristic of Bitcoin and PoW systems. It's not an inherent thing about Blockchains. Other cryptocurrencies don't have that problem.
I don't know about zero protection. Bitcoin-like currencies protect you from many things. One thing they don't protect you from is eBay sellers that scam you. Alternatives exist for this problem (namely money escrows).
Re: your last question, I think using a credit card is better today for buying stuff on eBay. But Bitcoin like systems solve a more fundamental issue which is that they let you and you alone control your own money. That is not essential for buying on eBay. But it is essential for other things that I would argue are more important. See my higher lever comment about Wikileaks funding, for example.
Wouldn't the real solution here be to push for the tech currently used by centralised banks to be updated so transactions can happen almost instantaneously? This would also come with the advantage of preserving the protection from hackers and fraudsters that centralised banks currently offer their customers.
Better bank tech exists in many places outside the US (pretty much everywhere, really). Better bank tech does not guarantee my ability to send you money, and it certainly does not provide that ability if any single one of a number of large private and/or public entities decide to preclude me from it.
Guaranteed access and censorship resistance. That is a clear advantage of cryptocurrencies at this time (not of blockchains, mind you), and I think it's a significant one. That doesn't mean you should use them.
If you are curious and up to the challenge I will give you an Argentina (where I currently live) bank account number and if you can get USD 1 in it by the end of next week I will send you $50 worth of the cryptocurrency of your choice.
PS Also I don't want to sound presumptuous but it seems to me you use the word "blockchain" as an equivalent to "bitcoin" or "cryptocurrency". It could be the case that some of the points in this type of discussion rely on the differences between those concepts.