B) Indirect taxes
Indirect taxation included trade tariffs, custom tolls, gate tolls, port tolls etc..
This was an important source of income for the Empire and custom tolls were legislated around 4th century and they were fixed on 12.5% for exports from Constantinople ports. Same percentage was fixed also to imports to Constantinople and the trade ports of Abydos.
It was such a major income source for the Empire what when Empress Irene abolished all tariffs and tolls the Empire's finances sunk and forced her successor Nicephorus I to reinstate them on the very first day of his reign.
Tax/tariffs/toll exemption was rarely given and only afrer a thorough investigation that the finances of the Empire wont suffer.
Exemptions were given freely during the Komneni era because of the bad financial situation of the state. In order to avoid destabilaisation of trade after exempting Venetians from tolls Komneni lowered all other tariffs/tolls to 4% in order to encourage other merchants to come over. But the combination of exemption and lowering the tariffs/tolls reduced the income of the Empire shattered its currency and ultimately destroyed its trade.
3. Banking System/capital market
This was an important sector of finance, trade and agriculture and banking system was governed by state interventionism in order to avoid cases of profiteering and usury.
Emperors enacted severe laws against profiteering and usury based in Church's teaching which prohibit them.
The majority of capital movements happened between professionals (goldsmiths etc.) and a few citizens who offered loans with interest to merchants, farmers, labour unions etc.
Until the reign of Justinian I the highest interest rate allowed for loans to people was 12% while in his Novellae the same Emperor instituted benefits when the borrower was poor.
People who had legally formed a "Bank" could loan with an interest rate up until 8% while private citizens offering loans could not exceed 6% on interest rate. Also it should be noted that wealthy landowners and nobles lending money to their surfs (or other citizens who approached them for a loan) could not exceed 4%
Of course several times these limits were ignored and lenders charged higher interest but they never exceeded 12% since Justinian's legislation allowed the borrowing of money with 12% interest but only for external trade.
It should be noted that when someone wanted a loan for sea trade the interest could climb as high as 17% (also allowed in the novellae) since sea trade was extremely dangerous (piracy, storms, mutinys etc).
Emperor Nicephorus I was forced to enact severe legislation against usury and profiteering since the Empire was in a finacial crisis during his reign and these phenomena were rampant.
However this legislation didnt stop him from lending money from the Imperial Treasury with 16% interest rate to everyone...

By 8th-9th century usury and profiteering were rampant throughout the Empire and it came in about 10th century and Basil II to enact legislation regulating interest rates to 8-9%.
However after Basil II and the gradual declining of economy Basil's laws fell into misuse and usury started again until 13th-14th century when it passed to venetian banks (which were exempted from Byzantine laws and could charge even higher interest rates thus destroyng whatever was left from byzantine economy).
4. State Industries/Factories
Of course as we said above the main sources of income for the Empire was agriculture and trade.
Agriculture was mainly for internal consumption (grain, oil, honey, fishing, flocks etc.).
Also an important factor for the economy was the Imperial monopoly of gold mines (Balkans, Egypt, Sardenia, Spain), copper mines (Balkans, Cyprus). lead mines (Attica, Kappadocia) , iron mines (Sardenia), sulfur and marble quarries (Greece) etc.
Several factories also were part of Imperial monopoly such as purple dye factories, clothing factories, weapon factories, tanneries etc. and it was strictly forbidden to any citizen to open a factory of these mentioned above or control a mine (at least not without a licence from the Emperor himself).
Private citizens could open factories like goldsmithing, jewelry, carpentry, pottery, furniture, glass works etc. but they were constantly overseen by the state and the labour unions.
Most of the factories working in the Empire were located in Constantinople and its suburbs and they were overseen by the Praefect of the City, while all provincial factories were overseen by the Comes Sacrarum Largitionum.
Production aimed mainly in covering internal consumption of goods and exports were rarely allowed (like wine which was exported to France mainly) and only from the ports of Constantinople.
It should be noted that exports of weapons, silk clothing, purple dye an few other products of Imperial monopoly was strictly forbidden under severe penalties.
Smuggling was severely punished and all exported goods had to be inspected first by state officials and if everything was ok the merchants obtained formal papers with the Imperial seal to export them.
One notorious case of smuggling happened in 968 when Liutprand (the german ambassador) decided to smuggle a few silk clothes to Germany without obtaining formal Imperial licence.
Customs officials found them hidden in a trunk and confiscated them after publicly humiliating him.
Thats for now... I ll continue later or tomorrow with trade...