To buy a plot in someone else's town you can go to the town and search for a plot that is for sale. Plots that are for sale will show their price in chat when you walk over them. You must join the town before you can purchase a plot in that town. Once you've met the previous requirements you can say
- /plot claim
Please note that if the town is deleted then you will lose all plots you own in that town and the gold will not be returned to you. Also the mayor can reclaim plots sold to you, in this case the gold used to buy the plot is returned to you including the cost of any wool and valuable blocks you had on the plot.