On mobile there is no global chat, so that's not a method for everyone. Mentioned already is to ask friends or even neighbors who they bought goods from. That can give you an idea of both prevailing pricing and reliability of goods sellers.
A guild can usually steer you to safe sellers, but ask yourself if your guildies know what they're talking about. Case in point: in a previous guild I asked about Arc goods and got 3 names the higher-ups worked with for guild GBs. Quotes were from 400 to 450 a set. I got a pm from another guildie I trusted and got a set from his guy for 150, fast and no problem. No one ever got ripped off by the guild recommended sellers (in the sense that they ran off with the fps), but people definitely got hosed on price. Kickbacks? I couldn't say, but I'm a suspicious cuss. In a good guild on the other hand it's not a problem and probably your safest bet for finding a seller.
I have gone with recommendations of people I trust, I have put fps out first and collected the goods after, and have never once had a problem. A couple of them were even willing to let me pay over a period of over two weeks if I needed to. I did not take advantage of this, as paying promptly (both parties) builds trust. If someone asks, I can recommend my seller. If I need goods my seller doesn't have, he can put me in touch with someone who does because he trusts me and I can in turn trust his recommendations. It's all about trust, good reputation and word of mouth.
If nobody's heard of some guy, proceed with caution lol. If the seller is among the top players on your server, it's very unlikely they're going to rip off anyone.
Edit: sorry to go so off topic! I agree with
@Mustapha00 that there's no reason you can't have both in your city; I've seen some cities with both.
@Axelroder, I'm curious about how you plan to use the space you might gain by reducing supply buildings. What's your game plan?