Couldn't tell you...don't know where you're located, but I'd sell locally if at all possible. Around here I post on local gun forums and sell FTF with a bill of sale if the buyer wants one. Most don't want paperwork and my state law has only 2 stipulations about selling a firearm so I just follow those. Your state may be quite different. Always abide by local regulations. You could also put it up on gunbroker...but I find those situations to be a lot more of a pain in the ass as well as less profitable since you have to pay fees at the FFL, to ship and to the hosting site.
Generally I'd agree with you about using gunbroker, but as you noted, given todays market (sellers market by far), Gunbroker would probably be my first choice to unload a higher end gun at a higher asking price.
I sold a clean Marlin lever manufacturered in 1899 for $1,800 earlier this year, and the fee from Gunbroker was around $72 (shipping not included). Honestly don't think I could of gotten that buy price locally as local guys were a pain in the butt haggling over the original $1,000 asking price (starting bid on GB was $1,100).
The key is to include your starting asking price with the gunbroker fee added in, along with buyer pays shipping (adding insurance as an extra as wellas an option to cover your butt if the postal service screws up, which I've never had happen myself).
Know nothing about the CZ Bren, but if $2,300 is what you want out of the gun, list it with a starting bid of $2,400 (add your FFL transfer fee onto the asking price as well PLUS shipping. Check with your local FFL on what insurance will run for the the starting bid price (I believe my FFL had like $500 insurance already in his shipping price, which was only like $45 to ship the rifle to the other coast).
Gun broker can be a pain to ensure you cover your butt on the final buy price, but in a sellers market, it's probably the most legit way to maximize the sell price to a national buying audience and it's basically free to list the gun.
Generally my FFL charges me $20 for a transfer, which I find more than reasonable and he has a brick and mortar shop, open 10 hours a day, 7 days a week. Not certain on Virginia, but I'm guessing the OP is paying the FFL fee no matter how he sells the gun now, even FTF.
For the XCR platform, love it. Decade or so ago, got my first one and loved it so much I picked up some others (all used, all run great and are reliable IMO). But I love the description of it being a "boutique gun" which I think is a perfect description for the XCR. Really wish RA would support those dealers more here on this forum who support and actually push the XCR platform as RA can really be a hit or miss per customer service and inventory on parts when you deal with them directly. For that reason alone I would always have some AR's laying around as parts and pieces are plentiful from various manufacturers (couple years ago when it was a buyers market I stocked up on lowers and parts for the AR, and glad I did).
To the OP, I might of missed it, but if you do sell the CZ, just make certain you have a source to get a XCR. Seems like pickens are slim pretty much anywhere. Also know what generation it is if you buy it used. RA kind of turned me off when they started redesigning parts for their latest generation rifle. I picked up a butt load of parts for my "older" XCR's, glad I did. Guys here could tell you more about what parts are and are not interchangeable between the new and old XCR, as I don't have the newest gen rifle. Just something to be aware of so you know what you're looking at before you buy one.
Since I know nothing about the Bren in that particular caliber, have to ask, what kind of mags do they use? Guess I'm old school as the only mags I trust for that round are steel, and I only use my AK's for that round.
For suppressors, I generally use an Omega and Specwar 5.56, and honestly if I shoot without ear pro (which is not hearing safe with any supersonic round), and really can't tell the difference between a XCR or AR on loudness at the shooters ears. With earpro, pretty much the same.