As a suggestion if you are buying used commercial traps, which I do recommend from a price perspective, look at them closely as they are not all created equal....
If you are buying used square traps there are small square and big square traps. If your puller, or your arm, can handle the wieght of the bigger squares (you have the space on your boat) get them as prawns will only fill a trap to a certain density so the bigger traps have a larger catch capacity. With the square traps the tops, sides and bottoms may be made from 1x1 coated steel mesh or 1x1/2 or some combination of the above. I prefer 1x1/2 as it leaves you the option of sorting out the smaller prawns rather than having them released for you as you pull the trap. Next thing to look at is the entry ramp materials. The better traps will have a finer mesh plastic 1/4x1/4 for the entry ramps. Check the adjustability of the ramp mouth opening. It's nice to be able to adjust the opening size. Some traps have a rigid mettle collar around the openings that makes them very hard to adjust. Look for the traps with a built in bait cage in the center of the square with a flip top opening. Good ones will have a peice of rubber tubing that acts as a spring to hold the lid of the bait cage down so you don't have to use string to tie them shut all the time. Last check all the crimped connectors that hold the peices together. There are different guages of crimp connectors and the thinnest can rust thru in only 1-2 seasons. A crimper, spare crimps bands and spare tire inner tube are a good investment if you are going to fish this type of trap so you can do your own repairs on the fly.
If you are buying the round commercial traps check the netting closely. The cord rots over time so give it a good tug all around the trap. Not a big deal if it's got spot holes as you can buy cord to patch them but it will let you negotiate the price. Some traps might need ALL the netting replacing if they are old or if they have been fished in an area with lots of dogfish....
For both types of trap look for ones where the lift lines have been woven into the sides of the trap all the way around the edge for extra strength and a balanced lift. Also look for rigged bait jars. If it comes with a jar rigged in already you just saved $5... if it comes with a quick disconnect you just saved a few more $$$
As for square vs round it's a bit of a religous debate but my opinion is that the collapsable steel frame round traps are better if you are tight for space on your boat but the sqaure traps fish faster. The commercial guys all used to use square traps when they could pull their traps several times a day. With the new regs they can only pull them once so they are not as concerned about how quickly they fill up. They are now more concened with how many traps they can put on their boat and which ones hold the most with a long soak so they have moved to LARGE round collapsables.
Make sure to buy both a line wieght AND a weight to put 10-15' in front of the last trap in your string before it heads up to your float. Without the bottom wieght the float will rock the first trap in the line and you'll catch very few prawns in that trap.
Hope this doesn't sound preachy but I bought/fished a lot of traps before I figured some of this out and I wished someone had told me things to look for out of the gate as I'm sure I could have saved a few bucks and caught more prawns....
Happy prawning..... and to answer the orignal question Sam at PRO in Powell River usually has a good selection of used commercial traps, floats and rope. (used prawning rope is usually a VERY good deal and is easy to handle as it's already very relaxed)