This is why I think an FSM is a good idea on APIs

main.go

package main

import (
	"fmt"
	"log"
	"net/http"

	"github.com/julienschmidt/httprouter"
	"github.com/looplab/fsm"
)

func newHouse(name, state string) *house {
	h := &house{
		name: name,
	}

	h.forSale = (state == "forsale")
	h.fsm = fsm.NewFSM(
		state,
		fsm.Events{
			{
				Name: "buy",
				Src:  []string{"forsale"},
				Dst:  "owned",
			},
			{
				Name: "sell",
				Src:  []string{"owned"},
				Dst:  "forsale",
			},
		},
		fsm.Callbacks{
			"sell": sell,
			"buy":  buy,
		},
	)

	return h
}

type database struct {
	houses map[string]*house
}

func (d *database) get(name string) *house       { return d.houses[name] }
func (d *database) upsert(name string, h *house) { d.houses[name] = h }

var db = &database{houses: map[string]*house{}}

type house struct {
	name    string // poor man's address
	price   float64
	forSale bool

	fsm *fsm.FSM // keep track of our state
}

func sell(e *fsm.Event) {
	name := e.Args[0].(string)
	h := db.get(name)
	h.forSale = true
	db.upsert(name, h)
}

func buy(e *fsm.Event) {
	name := e.Args[0].(string)
	h := db.get(name)
	h.forSale = false
	db.upsert(name, h)
}

func getHouseByName(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
	h := db.get(ps.ByName("name"))
	fmt.Fprintf(w, "house for sale: %v\n", h.forSale)
}

func sellHouseByName(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
	h := db.get(ps.ByName("name"))

	err := h.fsm.Event("sell", ps.ByName("name"))
	if err != nil {
		fmt.Fprintf(w, "unable to sell house: %s\n", err.Error())
		return
	}

	fmt.Fprintf(w, "house marked for sale\n")
}

func buyHouseByName(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
	h := db.get(ps.ByName("name"))

	err := h.fsm.Event("buy", ps.ByName("name"))
	if err != nil {
		fmt.Fprintf(w, "unable to buy house: %s\n", err.Error())
		return
	}

	fmt.Fprintf(w, "congrats on the house\n")
}

func main() {
	db.upsert("mike", newHouse("mike", "forsale"))
	db.upsert("taylor", newHouse("taylor", "owned"))

	router := httprouter.New()
	router.GET("/house/:name", getHouseByName)
	router.GET("/house/:name/sell", sellHouseByName)
	router.GET("/house/:name/buy", buyHouseByName)
	log.Fatal(http.ListenAndServe(":8080", router))
}

result.txt

$ curl -sXGET http://localhost:8080/house/taylor
house for sale: false

$ curl -sXGET http://localhost:8080/house/taylor/sell
house marked for sale

$ curl -sXGET http://localhost:8080/house/taylor
house for sale: true

$ curl -sXGET http://localhost:8080/house/taylor/buy
congrats on the house

$ curl -sXGET http://localhost:8080/house/taylor
house for sale: false

$ curl -sXGET http://localhost:8080/house/mike/sell
unable to sell house: event sell inappropriate in current state forsale

$ curl -sXGET http://localhost:8080/house/mike/sell
unable to sell house: event sell inappropriate in current state forsale

$ curl -sXGET http://localhost:8080/house/mike/buy
congrats on the house

$ curl -sXGET http://localhost:8080/house/mike/sell
house marked for sale

$ curl -sXGET http://localhost:8080/house/mike/buy
congrats on the house

$ curl -sXGET http://localhost:8080/house/mike/buy
unable to buy house: event buy inappropriate in current state owned