If a personal advert includes a TLA, it could be to describe the person who is seeking and/or the kind of person they seek; for example, an advert stating 'DLF for SJT' means that a divorced Latina female is searching for a single Jewish transgender person.
First letter
The first letter often describes the relationship state or sexuality of the person:
D: Divorced
G: Gay
M: Married
S: Single
W: Widow or Widower
Middle letter
The middle letter generally represents the ethnicity or nationality of the person posting the ad. Can be replaced by 4, standing for the word for ("seeking", "desires", etc.).
A: Asian
B: Black
H: Hispanic
J: Jewish or Japanese
L: Latino or Latina
W: White
Last letter
The third letter commonly describes the gender of the person (or couple, if that is who is seeking or sought).
C: Couple
F: Female
M: Male (or Man)
T: Transgender
W: Woman
List of miscellaneous abbreviations
As well as three-letter abbreviations of the format described above, a number of other acronyms and abbreviated words have been popular in personal adverts at different times and in different places. This list is far from complete:
ALA: all letters answered
ALAWP: all letters answered with a photo (probably means the advertiser will only reply to letters that attach a photo)
GSOH: good sense of humor
ISO: in search of
LTR: long-term relationship
MBA: mutual business arrangement
NSc: non-scene
OHAC: own house and car
PnP: party and play
WE: well endowed
WLTM: would like to meet
yo: years old (e.g. 23yo)
NSA: No Strings Attached