I personally wouldn't buy ANY of those used... you can actually get a better truck NEW than those.
Now, none of the ones I am going to list are top-of-the-line... but these will give you some ideas, and some of them are ALMOST race-worthy.
To start with, here are the ones you listed, BRAND NEW:
http://www.amainhobbies.com/product_...oducts_id/7328
Losi Aftershock, ready to run, $369.99 BRAND NEW and it is less than they want for the used one.
Without knowing WHICH MGT, I can't give you a link, but it is not a good deal...
Raminator:
http://www.amainhobbies.com/product_...ducts_id/20074
$429 new
NONE of those are good options... to start with, I suggest a Truggy instead of a "regular" Monster Truck. They are MUCH more capable, MUCH more durable (even the cheaper ones) and more popular for racing.
Here are some of the "low end" options... there are better ones than these, but these are not really bad:
The bottom end:
http://www.amainhobbies.com/product_...ducts_id/17279
Thunder Tiger stuff isn't bad... especially for that price. Just because it is cheap ($299) doesn't mean it is not durable.
Maybe a little better:
http://www.amainhobbies.com/product_...ducts_id/31199
Or this one:
http://www.amainhobbies.com/product_...ducts_id/10615
All of those are under $400, and all of those are much more capable racers AND bashers than any of the ones you listed.
For a few dollars more here are some more options:
http://www.amainhobbies.com/product_...ducts_id/12363
The RTR version of the truck I used to race for $509.99
http://www.amainhobbies.com/product_...ducts_id/33276
I wouldn't even consider a "monster truck" EXCEPT a Losi LST (NOT Aftershock) or Revo.
Also, something different:
http://www.amainhobbies.com/product_...ducts_id/26135
I hope that helps!