If you want your stall to be on your trading horse or camel:
1st. get your trader flag on (thieves can not summon transport in towns...

duh)
2nd. dismount and make your stall and put all the stuff in and CLICK OPEN!!! I HATE IT WHEN NOOBS FORGET TO OPEN THEIR STALLS!!!
3rd. after your stall is done simply click mount, and hooray, your stall is on the transport
P.S.
after those 3 steps, you can dismount again and your stall will move a bit forward and your weapons and shield will show (
SoSun weps and shield are a pretty nice attraction).