Okay here's some numbers that Larry Niven has come up with for interstellar, and in fact intergalactic, experiential travel times, they clearly assume fuel is free and infinite (which is theoretically possible but the tech involved could probably be used for FTL cheaper and easier) but they're for one G continuous acceleration flip over deceleration trips. The point is they make it clear that the crews are in for a long but survivable trip; 4 years to Proxima Centauri, 21 years to the centre of our own galaxy, 28 years to Andromeda. (As a note I'm reasonably sure that the longer trips at one G would actually violate the speed limit, C, but they're also unrealistic in the scope of the question in my view so I'm treating that as a non-issue for the purposes of this discussion). So crewing a STL trader is awkward but not too bad, so the question really is what kind of trade goods are valuable enough and keep long enough to be worth the hassle and good at the end?
I'm going to assume a "post-scarcity society" in which nanotechnology is limited and in particular biosynthesis is problematic, rare, expensive and/or nonexistent. I've also assumed that there are starships that can reliably bridge the gulf between the stars carrying worthwhile cargoes of goods. This leaves several categories of trade goods, Art and Historical pieces - the antique/museum trade, Information - this can be IP like the light speed datanet in Wil McCarthy's Lost in Transmission (the problem with this is that we don't really have a means of transmitting data across stellar distances, see this article, and in the novel it does break down) or it could just be the mail, and "Unreproducibles" - this covers a lot of ground (and technically includes art etc...) but it's things that we don't have the technology to make and that can only be found certain places; I've deliberately categorised Art and Historical artifacts separately because there's a very limited stock of such things. Unreproducibles are the most likely to be traded in large value transactions, this could be the Melange around which the Dune series centres, the exotic timbers of Plague Ship, or the gem trade of Uncharted Stars, the trade goods are complex materials who's unique physical characteristics can't be synthesised, that only occur on a single planet or a couple of planets, and which are stable over extended storage times. Potentially this may extend to extremely rare elemental raw materials over short distances from exceptionally rich sources to particularly bereft markets, but that's unlikely.
Provided that there is a demand market for these goods that is sufficient to pay for shipping costs over interstellar distances they can be traded at any speeds, but it's going to look a bit more like feeding frenzy than what we think of as a "market", with supply and demand. There is the possibility of a limited futures market in this format since the ship coming in will be able to broadcast their goods at lightspeed while arriving somewhat slower but the actual arrival of goods creates a supply in a demand market that hasn't had new goods for some period of time. Prices are going to be at their highest just before the arrival of new goods and are going to collapse for a time when new goods flood the market from a freshly arrived shipment. This turbulence in the market is going to be awkward for traders and buyers alike, especially with goods that are available from several places instead of just one, arriving just behind a competitor could be ruinous to a trade ship. So in effect any trade route is going to be a single shipper monopoly, not because of any deliberate conspiracy or controls but because anyone who comes in late is simply going to go bankrupt.
There is one other good that will always be worth something in any society, and is actually more valuable in post-scarcity environments; Time, any drug, any treatment, any anything that can give people a longer life is going to be worth it's weight, or probably much more, in any material you want to name, if such a treatment requires an unreproducible substance then you have a license to print money, if you can secure the source. There are any number of examples of this in fiction from Dune to The Collapsium, in James Blish's Cities in Flight series they use life extension as a currency in and of itself.
Which brings up an interesting point, there has to be a currency to trade in for all of this to be successful, ships that can cross the gulf between stars are going to be expensive in materials, maintenance, and crew. The trip itself may or may not cost a lot too, I would suggest that fuel is going to have to be basically free to make interstellar trade economically feasible but that's me you may go another way. All of this is going to have to be paid for one way or another; any currency being used is going to have to have purchasing power at both ends of the trip, so that what you, as a trader, earn at one end of the trip is usable for fuel and maintenance costs at the other.
Short version: You can trade in sufficiently scarce/unique goods at any speed provided you have a high enough demand for these goods, a currency to pay for them, and can ship them before they "go off".