I do like the premise and idea of this for the most part, however the hurdles you run into are the below;
- People don't need realtor companies.
- You're charged for something you do not need.
- There are very few realtor options.
- It takes 1 command to sell a house.
In real life, the process takes longer because you have to first get a mortgage (something which does not exist in GTAW), then you have to find a house to buy, of which many factors come into consideration, IE structural integrity, condition, location, size etc whereas on GTAW only two of these examples are applicable for purchasing a property. After you have found a house you want to buy in real life, you then have to negotiate with the vendor on a price, get the offer agreed, then instruct solicitors/lawyers from both sides of the agreement, get surveys for the structure and for the value of the property.
All of this in real life takes about 4-8 weeks depending on how efficient your solicitors, and agent/realtor are. In game, there is no need to get a surveyor to come and survey the house, there is no need for lawyers, there is no need to negotiate because if you do not buy it someone else will in many cases. With this in mind, a realtor has a 'value' because they can market the property online (the average person cannot do this cheaply), have a pool of clients looking to buy properties, and oversee the completion/sale of the property while you are looking/working for other properties.
In game you cannot easily simulate this same level of 'worth' for a realtor, advertising on the forums is free, advertising in game is a negligible expense, and there is no incentive to pay someone to do this for you. Also in real life, there are a lot more options and variables when it comes to house buying, whereas in game there is no running costs, or fees that you have to be attentive to.
While it's a good idea in theory, I think the only solution to implement something like this would restrict anyone other then realtors to do property/commercial for sale ads on the forums or in game which is very restricting and a bit over the top.