There is nothing wrong with buying second hand gear. You really don't need to read the rest of this. If you're on a budget and you really want a particular lens or camera, buy used.
There's plenty online about how to win auctions on Trademe and Ebay. But the three key things to remember are:
1) Don't bid up the price early. It's a sure fire way to inflate the price. Bidding early merely creates more interest and whips up a frenzy of panic in other buyers, many of whom are buying with their heart rather than their head.
2) Do your research. E.G. - Is the lens compatible with your camera long term? Third party lenses will often not keep up with your chosen camera maker's firmware upgrades. Nikon lenses made in the 70's still work on the latest DSLRs with some caveats. But a brand new Tamron or Sigma or other 3rd party lens, might not maintain all features with future Nikon/Canon bodies. Don't be afraid that someone is selling their gear. People change systems, some needlessly pursuing the latest and greatest, especially a dominating mirrorless system. But do ask questions about the item's condition. Although a spot of fungus in a lens shouldn't deter you, it might possibly infect your other gear in storage.
3) You can't lose something you never had. If you miss out because someone wants it more than you, move on and wait for the next one. It's crazy to bid against someone to only find you've spent a little less than buying new. Gear depreciates, especially camera bodies. But excellent glass will last you a lifetime if cared for. The longer you wait, generally the cheaper they'll get.*
I've bought most of my gear online, mostly from the U.S. where gear is cheaper new and the used market is flooded with excellent camera equipment that rich people discard for the latest thing. I also buy a lot of gear from Japan where they seem to look after their equipment meticulously and service is generally phenomenal. The Bronica above from etigoboy on eBay arrived within a week from Niigata in Japan to Wellington. It's coming on 40-years-old but looks and operates like new.
* Vintage gear worldwide is enjoying a resurgence in popularity and accordingly prices are inflating. Quality 60's and 70's Nikkor lenses for example which would have been taken to charity shops or discarded when digital arrived on the scene are now fetching top dollar, partly because digital video quality is so good that manual lenses are in huge demand, along with those shooting macro. Low light shooting has improved dramatically with full frame cameras so astro and night photography where autofocus is next to useless is also impacting on price to the extent that some lenses are fetching more than they were new. So there's plenty of competition out there but thankfully Trademe, which effectively only allows New Zealanders to bid still offers the odd bargain. Just make sure you set a budget and stick to it.