Safest way to do a deal is in-person, that way you get to meet the person.
If that's not possible because the item needs to be shipped, then Paypal.
Paypal offers both buyer and seller protection policies as long as you meet the requirements.
Ie. Sellers get protection if they mail to the verified address of the buyer
Ie. Buyers get protection if there's no proof of tracking.
ASC has the Trader Rating system which allows you to see who's safe to deal with, keep in mind this isn't like eBay so even one negative can raise some flags. But make sure you read the feedback and perhaps ask other members of the forum what the issue may have been about since alot of time unfair negative feedback gets left and the mods don't get notified about it.
You think it's not safe for customers to buy from 'retailers' or other individuals off this board? The retailers undergo more risk as they usually require several contacts to get airsoft into the country, there have been many occasions that retailers have been screwed by the importer or by their suppliers overseas.