‘Trade-in’ is a noun. It is something that is accepted as part-payment for something else. That’s why advertisers need to drop the hyphen when writing copy such as:
‘Up to £100 off any computer when you trade-in your old laptop’.
In this case, your old laptop would be the ‘trade-in’, so, in effect, the ad reads: ‘Up to £100 off any computer when you laptop your old laptop’.
You will actually need to trade in your laptop to take advantage of this particular offer.
Wordwatch Towers 
This topic is sometimes more knotty than it seems, depending on the verb involved. Some have the preposition built-in, so that the noun, verb, and adjective forms might all be the same. Or not. In the basic verb I just used (“build”), it’s “built up” as a verb (past or pp of “build”); “buildup” as a noun; and “built-up” as an adj.
As always, check the dictionary if any doubts. If the dictionary doesn’t have an entry for the verb, say, “pick-up,” then you can assume the correct verb is “pick up.” (You’d still have to check whether the noun should be “pick-up” or “pickup”; I say the latter, as in short for “pickup truck.”)
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Agreed — yes it is. I’m referring specifically to ‘trade-in’ and ‘trade in’ here. I have a few posts coming up on hyphens. I understand that American English is more likely to run two words together to make a noun (as in your example ‘buildup’ above) whereas British English is more likely to hyphenate. We’d be more likely to write ‘build-up’ as a noun, for example. But there are rarely hard and fast rules when it comes to hyphens. The Oxford Dictionary of English specifically states that it lists one option when it comes to such nouns, but this does not mean that other options are incorrect.
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