First, make sure your buying and selling activity doesn't rise to the level of being a dealer. What is that level over a period of time? The ATF is trying to redefine that that so almost anyone buying and selling their own personal firearms qualify. At a minimum, the firearms you intend to acquire should qualify as C&R and retained as part of your personal collection. Buying and selling your C&R guns too frequently or too quickly after purchase could raise a flag.
Second, with the onerous registration crap that HI makes us do, it having the license adds more work to selling a C&R firearm. Unless your buyers are also FFLs (dealer or collectors), they still have to go through the PTA process.
Third, when you purchase a C&R under your license, you have to record all the details of the firearm and transaction and have that available for inspection if the ATF shows up. It's recommended ALL C&R firearms be tracked in your bound book (which is allowed to be in digital format) even if not purchased under the C&R license. Basically, any transfers of your collection both in and out of your possession must be tracked and the records retained.
I found it best to handle local sales as a normal transfer (no C&R license involved). The license offers no real benefit and simplifies the transfer process if/when you also sell it locally.
The only real benefits I found using my license were:
A. buying a C&R from another state: it can be shipped directly to my house -- no other FFL needs to be involved, hence no additional transfer fees. Just send a signed copy of your C&R license to the seller the same way you would if sending them your local FFLs license.
B. having the ability to ship a C&R to any state without paying a local FFL dealer. That's especially important in light of the tighter restrictions shippers implemented lately. Most shippers are limiting handgun shipments to FFLs
C. No need to apply for a PTA for any C&R firearm including handguns. You do, however, need to have a current long gun permit (yes, even for C&R handguns). You use that when registering the C&R. Avoids the hassle of getting handgun permits. You'll also need some documentation proving the firearm is C&R qualified. The ATF documents list a wide variety that are specifically covered, to include ceremonial and special edition models. If the stamped date on the firearm makes it eligible, just show the part of the ATF rules that give the appropriate criteria ("over 50 years old," etc).
D. No ATF Form 4473 required when acquiring firearms under the C&R license.
The best benefit is being allowed to conduct interstate transactions without paying FFL fees. As long as you're transferring at least one C&R every three years, the cost of the license is a wash versus FFL fees. The real savings appear if you're an avid collector doing many transfers under your C&R license. That can save hundreds in fees. It really depends on how active your collecting activities are.
There's another benefit which may or may not apply. If you buy any firearm from the CMP (Civilian Marksmanship Program) and have no other record of live fire marksmanship training, the program accepts C&R licenses, even expired, to show proof of marksmanship.
The license can also help when applying for a non-resident CCW license in some states. I believe Maine asked for mine to speed up the identification and background check process.
There's no down side to having the license other than taking the time to renew, if you aren't doing transfers at least every three years, or if you're acting in a way which draws the ATF's or FBI's attention as I already mentioned.