The left command block(the command block that gives you the item)
/give @p[X,Y,Z,65536,lm=Xp] {Item}
The right command block(the command block that takes the Xp)
/xp -Xpl @p[X,Y,Z,65536,lm=Xp]

the red carpet is were your going to get the coordinates for the command blocks and to make it that one spot you need r=65536 but leave out the r= also it's optional to do the x=,y=,z= but it's more easy if you leave those out also if you having problems finding the item id's there will be a link and I hope this will help you make a easy xp item shop
Here's a example
/give @p[-1265,7,-409,65536,lm=5] 364 10
/xp -5l @p[-1265,7,-409,65536,lm=5]
this would get you 10 steak

All block/item Id's: www.minecraftinfo.com/IDList.htm

if this helped you please comment