The string forums are a decent place to buy from though there are scammers there too. There are many knowledgeable people there and putting something up that doesn't smell right will usually result in questions asking for more information. There are a few volume sellers with impeccable reputations there too and it's pretty easy to figure out who they are.
Retail racquets used by Pros are more likely to be customized racquets.
4 x IG Prestige MP, 70 cm, 376 grams, 386 SW, ALU Power @50