With no FTL radio, there is no possibility of checking current market prices on a destination world. The most recent prices available will usually be a few weeks old. For goods in constant demand there is little risk, but fluctuating demand can make shipping more exotic cargo unprofitable at times. Most of the safe cargoes are shipped in bulk by large companies, so free traders have little chance to undercut them. This leaves mostly the more exotic and risky cargoes for transport by small trading operations. Often the huge profits a free trader makes on one particularly timely shipment will be offset by losses on the next few shipments!
An important influence on the profit-making capabilities of free traders is the Memer species. With minimal life support requirements, Memer can often undercut traders of other species, and thus form the bulk of the luxury goods shipping fleet in areas where they operate. Memer ships are unsuitable for transporting fragile or perishable goods or passengers however, so other traders can usually make a living by supplying these services.
The lack of FTL radio also makes interstellar banking difficult. Traders often sell their cargoes for local currency, then purchase supplies and their next cargo. Some barter their goods directly into something which will be accepted as valuable on the next world they visit. Any left over profits can be exchanged for portable wealth, such as precious metals or gems, though even these fluctuate in value from world to world. Local currency in physical form can generally be sold off-world for some fraction of its homeworld value, but only if you can find someone who will buy it off you. The selling price of local currency falls dramatically the further you go from its world of origin.
Another option is to deposit the profits with an interstellar bank and obtain a debit smart-card. This sophisticated card effectively stores the carrier's bank balance encoded on it, and the user can obtain cash equal to the stored value at another branch of the same bank on another world. Exchange rates can be quirky, however, since they are set by the bank. Physically, debit smart-cards are plastic cards, with security data encrypted on to the memory chip. Forgery is impossible with current technology, unless the cryptographic keys are stolen from the bank. The disadvantages of a smart-card are that it can be lost, and cannot be used on worlds without a branch of the issuing bank.
Loan credit and insurance are also available for interstellar traveler, but interest rates and premiums are high, to cover costs of shipping information between branches of the company. Defaulters on large loans form much of the business for interstellar bounty hunters.
In-system trading, where viable, is much more routine. Two inhabited worlds within one system can easily share resources to the extent that one may produce much, or even all, the food used by the other. In such cases, fleets of in-system shuttles will be used to keep supplies flowing.