in some parts of the US they are also called colonial posts. the ones at home depot are typically too low to the ground. they are extremely simple to make. there is good money to be made placing them and leasing them out. charge extra for flier boxes, riders, etc.
make sure you are insured properly. make sure you know your local codes regarding digging and locating. i personally know of two ppl who have disregarded their local codes minmized the job that they were doing in their own minds and unfortunately hit communications lines.
tell the agents that you provide this service for that all posts will be placed or removed within 72 business hours or whatever you need to do to create a time frame that allows you to group orders by area so that you are not driving from one side of town to the other and back again in one day. this is the key to making this service successful.
myself i recommend billing the agency and letting them collect from each individual agent for multiple reasons.
this is a service that i recommend that you have an actual form that the client fills out with address and a signature. do not take the orders over the phone, demand documentation. it is too easy to put a post up on a wrong address because at least that way if you have documentation you will be able to charge for it and then you just need to sweet talk the upset home owner as you politely replace the divot out of their lawn and apologize while blaming the realtor
jeez i could write a book on this. many years ago my company was asked to do the same service for a very good client. i was very reluctant but ultimately we ended up starting a new company providing this service and it was a very good, very profitable business. if you service a fair amount of realtors it can be a very nice stream of income.
but seriously if you have a shop, a garage..some space to work from this is something that you can teach anyone who can operate a circular saw how to construct. unless you are going to order a hundred or more i would find someone and have them build them. constructing them in an organized way with a simple circular saw you can easily knock out 6-10 an hour...if you have a radial arm saw...BAM...
if you havent already done so try to estimate how many you are planning on having in the ground at any given time and have enough for a buffer so that you are not having to make posts at the last minute...before the market collapsed we couldnt make posts fast enough. and as odd as this is going to sound...make sure to mark them (i had a brand made that we hit with a sledgehammer to imprint our info into the posts) because there is a tremendous amount of theft.