If low tech worlds buy lots of imports, they are going to be having constant trade balance problems. With a unified currency, this will end up also being a balance of payments problem, and they'll end up currency-poor and have difficulty with liquidity locally. Unless they have some very compelling exports, that is. Their economic basis might be mining, agriculture, tourism as these don't need tech to be valuable; also handicrafts and arts in the margins. industrial goods will be at a big disadvantage, but they might fill simple niche rolls. Maybe they can't build starships, but they could provide plumping fixtures for them, for example, but they'd be competing with fabricators on price, so it's gonna be rough getting contracts at low enough cost to pay your workers.
This means you could and probably will encounter TL 15 stuff everywhere, but low TL worlds MOSTLY just have low TL local stuff in their shops. Economics suggests you should be able to buy low TL stuff at bargain prices with your Imperial Credits, but mostly I just ignore this and use list price. Of course you can mail order the high TL stuff from off-world but working local jobs for local wages on a low TL world will mostly limit you to being able to afford low TL products, unless you are just on the low TL world for a short time, in which case you don't have time to wait for your package to show up. Maybe you can afford cheap stuff coming out on high TL manufacturing worlds - but cheap and simple rather than the fancy nice guns PCs always want. High TL manufacture can be used to make the good high TL stuff, but also could be turned toward turning out cheap consumer products at very low prices for mass markets. Interstellar travellers will always have access to a variety of items they picked up along the way, some of which are high tech.
IMTU I assume there are a variety of things going on - with products available from other worlds, at higher or lower tech available depending on the local market (local demand, shipping lanes, politics, proximity etc), but higher tech products will tend to be higher priced. But goods availability depends on a lot of things; even at high tech, not everything is made everywhere. Some low tech stuff is also nice and valuable, and sometimes even rare.
TL does mean something different on an outpost where nothing is manufactured. Obviously. The manufacturing TL is basically 0, but that would be absurd to label it as zero, if it is for example a TL 15 research station. It gives entirely the wrong impression of what is going on.
You need to use common sense in applying TL to what the PCs have access to and what they encounter.