Basically it comes down to one question when trying to win an auction using sniping:
Do I really want
If the answer is 'Yes', then an online service with multi-server redundancy is the way you want to go ... and you'll probably have to pay for it.
If it's 'No', then use a program on your computer or a free online service, (the free ones don't normally provide redundancy), but be prepared that you might not win because of things like, eg. power outages, ISP is down, computer blows up, you're having hot sex and forgot to turn the computers power-saving off, the city the data centre is in suffered an ebola outbreak, the data centre turned into SkyNet, etc, etc, etc.
If you don't trust the online service (why would you pick a service you don't trust?):
- unlink your ebay account from any monetary account, (PayPal, bank, credit card, etc)
- turn off TFA (if you use it - I do) and only enter the auction/account details an hour or so before auction end
- after the auction, turn TFA back on or change your password if you don't use TFA