draft horse collar question

I hear about the importance of buying a new collar so it can conform to a specific horse's neck properly. I've heard swapping collars is like (humans) swapping shoes, and you're setting the horse up for a sore neck. I'm assuming a used collar is okay as long as it is not totally compressed so it will still have some ability to conform? Sound logic, or faulty? I need to save $ where I can, new stuff is pricey, but I don't want to sacrifice my horse's comfort.

While in theory, that sounds completely logical, that's not something I've seen in practice. Most people start the year with a dedicated collar for each horse but as the season goes on and the horse looses weight, gains muscle their necks can change dramatically. Not many can go out and buy a 2nd or 3rd set of collars as their horses change. What I see at our auctions is the professional drivers buy up a wide range of used collars to fill in with. I think it's more important in the selection process to have the right size and the right fit (regular, half sweeney or full sweeney) over new. Check to see if the straw is not broken (it will flop open when you unbuckle it) and it's "full" and even. Pads are another issue. I cringe when people buy armloads of dirty, sweat stained smelly pads. Spurge and always buy new. Pads are 2" bigger than the collar size. If you use 24" collars, buy 26" pads.
I can buy a good condition used (draft size) for $30-60. New 2nds $50-90.

Agreed^^. We have always used the same collars on different animals. Just so long as the fit is good, you shouldn't have a problem.

A well made collar that fits properly will be fine for your horse, regardless if it is used or new.
The fashion in which the collar sits on the horse's neck does not lend itself to forming a mold so to speak.
Using the human shoe analogy, it would be more akin to "switching big mittens" between folks, as the collar should be allowed to move some, even in draft.
Brodhead Collar Shop has plenty of inexpensive new collars that will last for years with light work. Try the #60 tick face collar

I just picked up a used collar. It's all leather, but it seems very stiff to me. I have no frame of reference, so I'm wondering if that's just the way they are. I can't compress it with my hands hardly at all. I realize it can't have too much give b/c of all the force it is built to withstand, but just how stiff should they feel?

Yep, sounds about right . If it's soft or mushy anywhere, then it is either broken down or defective.

Yup, should be firm/hard.

You can interchange collars just fine - as long as you make sure it fits the shape and size of the neck... no different than how the same saddle CAN work for a few horses (as long as the fit is good)