There is the selling section but there is a post limit to get in and it is not the most highly trafficked area. Self promotion threads of the blatant variety are as mentioned rather frowned upon, but you do have a signature for a reason (guests can't see it) and emphasis might be on blatant (though conversion rates might be lower if they have to see it on the guide for a site linked).
It can also vary what is modded -- you want to solder something for someone then that is one thing (do also note every single modder that has ever been dragged before the beak has been because of them shipping ROMs with it, despite what the puff piece from the companies involved might focus upon). Modding copyrighted software for pay tends to be frowned upon for reasons of self preservation -- between take the money and run, cost disputes* and money makes lawyers perk up.
I have never seen anybody offer it here but the principle in games in general exists, and there have been instances I have heard of elsewhere in the world... "buy my course"/"I will train you". There is also a reason the ebegging sites for various emulators and modding crews might end up not inconsiderable.
*there being three major tiers. Beer money, basic technician levels and what rates are if I as an assembly capable embedded device botherer wanders over to sort your factory out. Many have troubles when the levels collide and drama is generally best avoided.
Personally I do nothing of the sort, offer nothing of the sort but that is more because I have no idea who would actually pay for anything like this. That is my problem to ponder though.