Leo,
I agree with Pepe that you can advertise your requirement far and wide, provided you post to the GPE which is FBO, despite the use of the word "single". However, I think for everyone perusing this thread, that while you can also post to GSA (assume e-buy), SEWP, etc as you suggest, you have to know what you are doing. I believe that market research should inform you on the method, generally I think of "open market" as triggering the FBO requirement, but if I choose to order through GSA e-buy, then that's another matter. See FAR 8.404(a). I generally teach new 1102's that market research and the requirement should inform them whether they do open market or utilize GSA schedules. If GSA, then use e-buy (and not FBO), generally speaking.
Our Small Business Specialist will sometimes advise us to cast a wider net and post a GSA e-buy requirement to FBO, but we have developed specific language for those scenarios so that industry is made aware that we intend to issue an order against a GSA schedule, so any responses through FBO should include their GSA Schedule information and we also post to e-buy.
Generally for MACs/GWACs, you could post to FBO, but you have to make sure you provide a fair opportunity to those contract holders (16.505(b)(1)( i )). This gets complicated if you don't know what you are doing because generally you wouldn't synopsize a requirement that could be satisfied through a MAC order. Without caveats on this, I'm not sure what you would do with an FBO response to something that was also done as a MAC fair opportunity if you didn't go through all the right steps. Of course I'm talking about posting or issuing a solicitation, and not RFI's or sources sought notices.
Again, I believe for the most part, market research should inform you on the method before you get to the issuing a solicitation stage of the game, and post it in numerous places. It may tend to confuse industry more than it generates interest.