well, first you have to define what is meant by "tech level". if the simple presence of goods of a certain tech level means that that location is at that tech level then yes, there's no reason to distinguish tech levels between trading star systems. but saying tech level represents local capablities seems to make more sense.
(how that locality is defined is an issue - planet earth is tech level 8, but tropical africa is fairly described as being tech level 1.)
if local capabilities are the definition then there are any number of reasons why tech levels will vary. drive, ambition, religion and other kinds of value systems, education, local wealth, government, corruption, property rights or lack thereof, life satisfaction, greed, war, resources, available free time, competition, and any number of other factors may bear on local tech level variation.
as for purchasing tech from elsewhere, simple cost is not the only issue. first, the item must be paid for in terms that the seller recognizes. a computer manufacturer isn't going to accept a dozen chickens as payment. two, the item must have support. a mediaeval peasant may buy a computer, but he'll have no power outlet for it. three, it must be useful to the buyer. a mediaeval peasant may buy a computer, but he won't have much use for it.
(how that locality is defined is an issue - planet earth is tech level 8, but tropical africa is fairly described as being tech level 1.)
if local capabilities are the definition then there are any number of reasons why tech levels will vary. drive, ambition, religion and other kinds of value systems, education, local wealth, government, corruption, property rights or lack thereof, life satisfaction, greed, war, resources, available free time, competition, and any number of other factors may bear on local tech level variation.
as for purchasing tech from elsewhere, simple cost is not the only issue. first, the item must be paid for in terms that the seller recognizes. a computer manufacturer isn't going to accept a dozen chickens as payment. two, the item must have support. a mediaeval peasant may buy a computer, but he'll have no power outlet for it. three, it must be useful to the buyer. a mediaeval peasant may buy a computer, but he won't have much use for it.