So, my used/new rundown:
Mask - buy new. The silicone is going to be better on modern masks. Although used masks are a dime a dozen they're usually really old. Spring for the 30-50 bucks and spend an hour in a dive shop with someone who wants to get you a good fitting mask.
Snorkel - used. You won't really be using this, and perfectly good used snorkels are EVERYWHERE
Fins - used. As you'll see, price for scuba gear drops dramatically once it's been used. You can find a great pair of fins for almost nothing. Just make sure that they aren't more than 5-10 years old, don't have cracks, straps are in good condition, not bent funny, etce
Weights - buy used. Can easily find them for less than $1 a pound on craigslis
Wetsuit - used. People buy and change styles all the time. There are good deals out there, just know how it should fit. It might take you a few purchases and a few dives to get the perfect one, but in the mean time you can flip them (buy low, sell higher) until "the one" comes along.
Hood, gloves, booties - i've had a hard time finding these good and used. Gloves wear out quickly, hoods are often improperly stored, booties have other people's feet in them, not to mention that's where all the urine ends up.....Look to Amazon. If you have a Prime account, you can buy a bunch, try them on, return shipping is free.
Dive accessories - used. I'm talking compass, clips, belts, slates, knives, clips, bits, bobs. You can usually find someone selling a box-o-junk for less than $20.
Knife - used if you can find it. Good small knives are hard to find used. You can find them for really cheap online, and you're gonna loose a few. I'd go stainless and just maintain it.
Computer - used if you can find one that has all the features you want. I've been hunting for a Suunto Mosquito for a while, but can't find it
Gauges - used if you need them. You're computer might take care of all of this for you, especially if you have a transmitter. If you go analog, go used. People give these things away, especially if they're attached to a regulator set.
BCD - if you are ok looking like you're from 1993, go with used. You can find great deals out there, and the cost of service plus a low purchase price won't
exceed the cost new. If you can find a used BP/W setup, go for it, at least for the backplate, harness, and tank adapters. A newer but used wing with out-of-warranty service may exceed the cost of a new one.
Regulator set - this is where you're going to do your math. You can find "great" deals all over the place. People seem to be giving these things away, especially if they're more than 10 years old. The problem is the out-of-warranty service and cost of service parts. So let's take a couple examples (this is what I am going through right now at the moment with some FREE gear I just acquired):
1. Aqualung Conshelf 22: 1st, 2nd, Octo for free: Because this is an older regulator, and hasn't been in service for at least 5-10 years, it needs a complete overhaul. At my local dive shop this is $90 ($30 per stage) + parts. I have to pay for parts because I am not the original owner and didn't buy from them, and the set is out of warranty. Thankfully this is a common regulator, and parts are readily available at about $30 per stage retail. So that's about $200 after tax to get it up and running again. Because this is out of warranty and I didn't buy from my local shop, annual service will be $40 plus $90 in parts again at a cost of $130 per year.
2. Aqualung Titan: Titan 1st, 2nd, and ABS Octo for ~$500 from my LDS. This is nearly identical in function and service parts as the used Conshelf 22, and comes with a lifetime warranty with free service parts. Aqualung requires a service/overhaul/service/overhaul annual maintenance schedule. My LDS charges $30 for labor for a service, and $60 for an overhaul if purchased from them.
Assuming I follow the same service/overhaul/service/overhaul schedule for both regulators
Year Conshelf Titan
1 $200 $500
2 Svc $330 $530
3 O/H $530 $590
4 Svc $660 $620
So you can see that after only four years, the new regulator set will cost less than the used regulator set because of the high cost of out of warranty service for the Conshelf. This may be different for you if you can find different prices for regulator services. I've done something quite a bit different though. I got the Conshelf set in a box-o-junk that also came with a "vintage" Mares MR12 III regulator set with gauges, all for a steal because I was able to show the seller the extremely high cost of service, and he just wanted to get SOME money for his old dive stuff. I sold the Mares set on eBay within a few hours for much more than I bought the entire set (some guy in Tavernier must have a vintage addiction), and when I've sold off everything else that I don't want I should be about $100 ahead. That will pay for an overhaul from the people over at ScubaToys.com, and from then on it's another $100 a year for a complete overhaul. That's the lowest price I can find, and costs about as much as all the service parts would cost if I purchased them and did the service myself (assuming that I can).
So I guess the end point is, whether used or new is the best deal all depends on you. If you have a LDS that does inexpensive service, or you are willing to do the work to hunt around for great deals, then used might be the route for you (especially if you don't know how much diving you'll be doing). If you just want something now, and KNOW that you are going to be diving a few years out, then new might be the better option. Because I made a lot of bad purchases that I was able to sell for more, and because I've been willing and put in the time and effort to buy and flip gear I have a full dive set and I'm ahead money. Granted this has been spread out over a couple of years and I've been living in coastal areas with a lot of craigslisters. As always, YMMV.