There are a lot of scamemrs around and i get added all the time by people who are not only on an account that was just created, but also by people who have 3+ scammer tags. Its unavoidable because by having a post on tp/outpost you're inviting any potential scammer to your account. Personally I always accept the friend request (unless their already marked), this way i can get another scammer marked and with any luck help prevent future scams.
Just a general rule: ALWAYS be careful when someone adds you (or when you add someone for that matter). Just because someone made an offer on outpost/tp doesnt mean their legit.
Also, one thing i've noticed is that scammers are getting smarter and aren't making ridiculous offers as much. So, just one more thing you have to be careful about.
A few simple rules to prevent getting scammed:
--only deal with reputable people. (check their rep for important traders posting rep).
--take a look at their friends list--see a bunch of scammers? or a bunch of SR donators/admins/trusted sellers.
--Always think about the offer and think about the legitimacy of the offer. Was the offer higher than the buyout, or even the same for that matter.
--Always check the person's profile and steamrep yourself. (dont just trust him and assume the link he sent you was actually his profile/SR).
--Another thing, which is often overlooked, check their recent aliases, reliable traders usually don't change their name that much, and when they do it usually isnt a drastic change. Scammers on the other hand will often change their name to someone who's well known (aka impersonation).
--And finally my favorite form of proof that you wont get scammed--an old account with a lot of games. Generally scammers have new accounts with very few games.