I wish folks would actually read the license that comes with media.
Below is the first paragraph of the TGC Game store license:
This document represents an irrevocable offer made by the Seller (as defined below) to the Customer (as defined below) to enter into a legally binding contract on the terms described below.
This offer is irrevocable and may be accepted at any time
by the Customer notifying the Seller of its acceptance of the offer. Notification may be made by sending an email to the email address below with the full name of the Customer in the subject line followed by the words "accepts the commercial licence terms for ...
whatever model you purchased".
Yes you can use the models commercially but you must send off the e-mail to do so, dont go on at me I didn't write this stuff.
I am also willing to bet this gets overlooked:
Retrictions
The Customer represents to the Seller that the Customer is either a body corporate or an individual of at least 18 years of age.
Since under 18's will be using parents to purchase assets I actually have no idea how the purchaser stands as regards to user (the designer). Since your not going to be creating anything commercial (well not mass production anyhow, if your selling ten copies to friends its hardly commercial)and till your able to set up an account to place your sales cash into and sell a zillion copies its all pretty much to cover any real companies with a fairly generic license.
Bottom line: yes you can use the assets in other engines, but if you want to use it commercially, you need to accept the license first.