In those cases I'd just recommend sticking to sellers who are established/reputable, or have return policies. Getting a quick authentication before paying is fine, but generally I'd say it's probably not very fair to win an item and then tell the seller you're not paying without an outside authentication first, if it's going to take awhile. Especially if you're going to need to ask for more pictures for the authentication, etc.. after you've won. Which is often the case with low-priced ending items. Those listings I find are often missing information that make most buyers feel safer in bidding a lot of $.
You can of course refuse to pay though, there is nothing a seller can do to force you, except open an unpaid claim against you after a couple of days. Enough of those, and it will affect your eBay account status.
While I understand that posting on AT (authenticate this) threads risks drawing attention to listings and driving up prices, there are authenticators who won't look at completed listings. They will only do active listings as a way to encourage buyers to abide by ebay policy in asking questions, requesting additional pictures and doing their homework BEFORE bidding and buying.
Allowing a listing to end and refusing to pay unless and until an item is deemed to be authentic is a violation of ebay policy. Bids and purchases are contracts by which you're expected to abide.
Not following through can not only get you suspended (as mentioned by lshcat) but can end you up on BBL lists of sellers who share non-payers and PITA buyers.
Don't do what I did haha. You can read my old ebay thread. I thought I scored a good deal on a Celine bag and snatched it up before getting it authenticated. Let's just say the entire ordeal was a NIGHTMARE. Save yourself the heartache and potential loss.