How would I go about finding someone selling a chainsaw through AS? I would trust that much more easily.
There is an Auctions tab as the previous poster stated, and then within that there are "auctions" where you bid on items, and "classifieds" where prices are fixed. You'll figure it out if you go there.
Just a word of caution, anyone can join Arboristsite so the fact that someone is a member here does not guarantee a good transaction. If you find a saw that you are interested in, look up the info on the member to see if they have been around and post from time to time or if they are brand new. The site does not provide any guarantees like eBay does if a transaction goes sour, and they occasionally do but finding a saw from a respected member here is more likely to be satisfactory than taking a pot shot on eBay.
The best option is to buy a saw in person so you can make your own determination. I don't presently have any "large" saws for sale but if your future wood cutting is going to consist of bucking logs, then as an inexpensive option you may want to consider a large older saw. Some reasons:
Older saws cost less.
Older saws don't have chain brakes. This will be less of an issue for just bucking than if you are working in trees limbing them.
Older saws don't have good anti-vibe (most) but it may not matter if you are only cutting an hour at a time and spread it out.
Older saws are heavy (most) but it will not matter as much for bucking purposes
Older saws are readily available because they have low appeal to the masses but something you are likely to encounter here on AS.
Older saws are hard to find parts for...With this site and eBay, that is not as much of an issue unless you get something really rare.
For example: I recently sold a McCulloch PM700 for under $200. That is about the 70cc class as has been mentioned. For a modern saw in that cc range you are looking at used prices of $400 to $550 with some either side of that and new prices starting at $800 for the most part.
I have some older running Mac 10-10 models that will be going on the block soon and those saws are quite inexpensive and more powerful than what you have. (<$100) No anti-vibe and loud but they cut well. Simple to maintain and lots were made so parts can be had either nos or used.
Regarding the questions on the splitter, these big "knots" are very challenging to split by hand. Probably can do most of it by manual means but don't "fight" with the tough stuff, cast it aside and go after it with a powered unit later.