The problem with buying used gear is this: Every piece of dive gear takes a sample of your DNA when you first take it on a dive. The moment someone else dives with it, it immediately stops working. That's why you must buy your gear only at your local dive store, so they can calibrate your gear to your DNA.
No, seriously: I use a lot of used gear. My new equipment is my wing, mask, snorkel, fins, knife, and lights. Everything else - my regs, computer, and drysuit, etc. are used. I was able to get my complete used set for about $1500, including regs, drysuit, tanks, weight, etc. If I have to sell all my gear (a distinct possibility) I would want to recover a significant portion of what I spent.
The prices I've paid are way higher than what other people on these boards will pay. You're from Ottawa, so you know that Canadian prices are higher than US prices. I'm from Vancouver Island, where we pay an "islander tax" on everything that gets barged over. For example, the FLDS here sell the Dive Alert Plus for $160.
Consider your local market, and don't forget that Customs WILL throw duty on anything you order from the US. Make sure that you're not buying "vacation-duty" gear. I'll imagine that you'll be doing a fair bit of cold water diving in those Great Lakes. The gear may work just fine with your bare hands, but you'll have trouble with it after putting on a pair of 5 or 7mm gloves.
When I bought that used gear, I was a student with my first kid on the way, so money was quite tight. Yes, it is life support equipment. In a sense, so is a car. Take it for a test drive, get a pro to check it out, and if it's a good deal, buy it.
The regs have been wonderful, except for servicing. (Nobody in town services Poseidon) I'll probably have to start using a mail-order service centre. Make sure the ones you buy are good for those cold Great Lake diving trips. Also, find out if the person you're buying them from gets cold sores. (I don't.)
The drysuits (I've owned three used suits) have sucked. One was way too leaky and the seals were going. Another seemed to fit okay, but had problems at depth. The "keeper suit" is nice enough, but it's taken years to get it leak-free. I have learned a lot about taking care of drysuits and how to patch them. I got a good price, but I'd like to get a brand new suit one of these years. (My current suit is so flexible that I can zip it up myself, and it's not a self-entry.)
The computer is freaking garbage. It's hard to read, it's failed more than once, the interface is obnoxious, and the manual is almost totally incomprehensible. (And I'm an Electrical Engineer!) I'll end up replacing this with a wrist-mounted system and putting an analog gauge in its place.
Used weights are great, especially if you find them on a dive. Bring a bag on every dive - you never know what you'll find. Lead doesn't go bad.
The MEC at 366 Richmond Road in Ottawa has great Wekona knives with a spectacular warranty. Stainless (304) is $21; Titanium is just $43. The point is that new knives are cheap -- keep that in mind.
In my case, I've managed to charm my way into several free lights, so it's not a typical scenario. If you buy a used Pelican or Princeton Tec light, they will take care of you in unbelievably amazing ways. For both companies, my experience is that they consider your breaking the light to be a failure on their part.
I bought my fins on eBay for $100 less than what they would have cost in town, and that's AFTER shipping, duty, exchange, and other extra costs.