I've been trying to get the GE working on my 2009scape singleplayer for a little while now and thought I would make a post to see if I could find some help.
I started by enabling the cheat in the config "i_want_to_cheat" which claims to allow the buying/selling of items at any price on the GE. This does allow me to buy any item at any price, albeit it is quite buggy. Search for an item, select it, get an error message saying I can't buy it. But if you set a quantity and price it still sets the order and it will be fulfilled with time.
I'm having trouble selling items though. Trying to select anything in my inventory gives me the "You can not sell this item..." error. I thought maybe trying to set a quantity and price would still set a sell order anyway, just like the buy orders? This was not the case, as it will set to a buy order instead when placed. I also tried running the command in game, ::allowge, but it doesn't seem to do anything as far as I can tell.
I've had some people tell me to just use other alternatives such as selling to vendor + giving yourself the difference in coin, just playing w/o it, etc. but I honestly would prefer the convenience of the ge interface.