I'd like to ask to provide instructions somewhere at process of purchase: the thing is that you can deal with trade offers only through last device where you logged at steam, which implies that if you have to login through browser at dispenser for purchase, you can't accept trade offers through client anymore unless you restart it (otherwise you'll be just getting error all the time). I think with new system, it might be crucial for people to know this.
Also it's good idea to try to accept them through browser because it works better overall and sometimes even when you get error, it's actually getting through after you refresh the trade offers page.
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