One suggestion for this strategy... the reasoning I think will seem clear enough. Remember that you have to PAY some maitnence on every city improvement (playing the sci-fi game in civ II: Test of Time, where you can build workshops... the factory equivalent... after getting primitive machinery VERY, VERY early... think iron working early, taught me this very well). During the Great Library age you have you have maximum taxes and minimum research. Since you have to always pay on city improvements and you have full taxes and no science, build marketplaces first (no need to rush them... you'll actually lose money overall if you rush them). Since your tax rate comes as so high, you'll multiply your income with marketplaces (read the civilopedia entry if needed for the reasoning here). When the Great Library age starts to come to an end... i.e. education comes near... build libraries. Hopefully, your libraries will finish about the same time as the GL turns off. Then, you'll have the multiplier effect going for your science, since it will come as so high. Then build any remaining libraries and your universities BEFORE building any more marketplaces or banks (I wouldn't recommend selling anything). Sure, this might seem like it costs more... but it really doesn't, I'll try and explain.
If you have no taxes whatsoever, your markeplace does nothing, but cost money. Similarly no science, libraries do nothing but cost maintainence (why heavy warmongers never should build libraries?). I haven't fully checked on civ III, but I think if you have 1 commerce going to taxes, your marketplace will do squat, but it still costs money (bad marketplaces). I know civ II rounded down. If civ III rounds up, the marketplace will then produce one extra gold, which means the marketplace just pays for itself, so it still really just sits there and does nothing. If the city produces two gold prior the markeplace, you get one extra gold, so again, the marketplace does nothing but sit there waiting patiently for more commerce. If civ III rounds down, for a 3 gold city a marketplace again just breaks even. Only when you have 4 gold from commerce before the marketplace will the marketplace make you money... and then only one gold. So, if you have your sci. rate at 90% or 100%, probably also 80% and to a lesser extent 70%, (given no luxuries) even marketplaces will either cost you money, break even, or do very little. The reverse works for libraries, except that 1 extra science unit comes into play at 2 beakers... you just basically convert your maintainence cost into an extra beaker this way. So, libraries and universitities end up as more important for this sort of strategy, as soon as the GL ends. Of course, you'll want some marketplaces and banks so that you can bulid stock exchanges for Wall Street, once you have hospitals or large enough cities that even 10% commerce makes commercial improvements worthwhile, and for those few turns when you can adjust that science slider back to 60% or so and still get your technology in N turns. Of course, what works most effectively will vary depending on city size, commerce level, and so on... but consider if you have a city at size 12 producing 26 commerce, and you have your science slide set at 90% (fairly generous high middle age condition). Then you'll have 2.6, or, 2 or 3 going to taxes (depending on how the game rounds), with the rest going to science. Either way a marketplace either costs you or just sits there. A library will give you 11 or 12 extra science, and university will compound that effect (50% of the original 23 or 24 beakers... or 50% of the 32 to 36 beakers you produce with the library??? I don't know). Of course, for a metropolis with 40 commerce, a marketplace makes cash at even 10% taxes. But, depending on commerce bonuses, many cities will need a hospital for that.