I'm not an expert, but I do have experience buying and selling CDs with four different brokers, one of which is VBS. Yes, there are significant inventory differences. Each broker cuts its own deals with the issuing banks (there may still be identical offerings from different brokers). The secondary market offerings tend to show more variety, because, in effect, a bulk purchase by one broker eventually shows up on the secondary market at multiple brokers. Some (including VBS) carry callable 15- and 20-year CDs, while at least one of the full service brokers carries only non-callables of 10-years or less. I've never discerned any rate differences in new issues. All brokers with which I have experience sell new issues without commission -- they make their money in the deal with the issuer.
I believe an important difference exists if you ever need to sell your CDs. I found the full service brokers to be unenthusiastic about selling, and I have anecdotal evidence that they got inferior bids compared to VBS. The "full service" process goes like this: you call them up to ask them to put a CD out for bids, they call you back later when you're in a meeting, you play phone tag for a while, and then you finally get to hear the bids and decide whether to accept. On a number of occasions I declined the bids, but on one interesting occasion I declined the bids and argued that the CD ought to be getting better bids. The next day they called me with a bid that was about 2 points higher. Since this was a $100,000 CD, that meant a few thousand dollars more for being a thorn in their side. Note: declining bids didn't get me a better price, declining and complaining did.
Another anecdote is that I tried several times to sell a CD through a full service broker and got no bids above par. I transferred the CD to VBS and they put it out for bids, got six bids, and the top bid was about 2 points above par. Again, that was a few thousand dollars for my trouble.
A final point of interest (thank you, I'll be here all week) is that when I sold that CD through VBS I immediately saw it offered at the VBS bond desk for 0.7 points above what I had sold it for. To recap: no bids above par at full service broker, sold for about 2 points above par at VBS, and then on offer at about 2.7 points above par. Ah, the life of the bond trader.
In case you're wondering why the heck I'm buying and selling long-term CDs, especially given these bid-ask spreads: as trustee and executor for some elderly persons, I purchased long-term CDs in their trusts, planning to liquidate them by death put upon their deaths. When the owner has only a short time to live, you might as well buy 20-year CDs to get the highest rate. As it turned out, interest rates dropped after everyone said they had nowhere to go but up, so instead of exercising the death puts I sold them at premiums.
Finally, VBS has a nice feature that allows you to check with a single click whether any bids are out for the CDs you hold.