Transsexual
About six months ago, on th news in my city, the news channel had a top story about LE fabricating things to create people to break the law.....this instance, a young very very good college boy was horny as hell and surfing CL. He found a girl who would have sex with him for party fun. Well, he was not that type but found it, showed up and got arrested. Thats wrong, so I say always be cautious. The above is crooked and not right, done only to increase revenues for the city and pay for the Michael Jackson funeral 
ha ha, its really not far from the truth.
Based on the anecdote that particular kid got screwed over by a dumb mistake but it's also worth taking into consideration that genuine drug dealers of all stripes are typically very willing to trade drugs for sex so posting that kind of fishing ad is very likely to snag real dealers which has the potential to lead up the narcotics chain which looks good for the police in a way that prostitution/solicitation arrests don't.
Despite the fact that LE can be pretty crooked it doesn't make much sense for the po-po to pose as a transsexual for any reason(drugs or sex) when it might net 2-5 busts in a day and a priced right ad from a non-ts female could probably net 15-25 in few hours.
Granted It's good to be cautious and never let your guard down but I think gentlemen that patronize TS girls are much safer from legal issues then gentleman that patronize non-ts girls.
Just my two cents