1) The Me'iri, on the Mishnah on 19b that says one may not sell to them anything that is fixed in the ground, writes that everything that is Mechubar to the ground is like the ground, so Chazal made a Gezeirah that if one sells a tree one may come to sell land. It seems that when the Me'iri writes that anything that is Mechubar to the ground is like the ground, this does not mean that mid'Oraisa it is equivalent to the ground, but rather that since a tree is similar to the ground there is an Isur d'Rabanan to sell a tree, because if one sells a tree one might come to an Isur d'Oraisa by selling the land itself.

2) The Chazon Ish (Shevi'is 24:4, end of DH v'Hineh) writes that when one sells to the Nochri a tree, one is selling the "Yenikah," the tree is drawing nourishment from the ground. It seems that when one sells the tree this means that in effect one is also selling something of the land since the tree takes its sustenance from the land.

3) It is worth pointing out that the Mishnah in Bava Basra 81a cites a dispute between Rebbi Meir and the Chachomim concerning someone who bought two trees in his friend's field. Has he thereby acquired any of the land? Everyone agrees that if he bought only one tree, he does not acquire any land. This strenghtens your question, that when one buys a tree one does not acquire the land beneath the tree!

4) It seems to me that this also helps us to understand the Me'iri better. Mid'Oraisa one did not give them a camp-down in the land when one sold them a tree, but since one did do something very similar to selling them land, Chazal made a Gezeirah that if he sells them a tree, he might come to sell them land.

5) The Ran (on the Rif) gives a different reason for why one may not sell them anything which is Mechubar l'Karka: "Do not give them any Kevi'us" in Eretz Yisrael. A tree is Kevi'us; it is something fixed. It seems that, according to the Ran, it is an Isur d'Oraisa to sell a tree because Lo Techanem is expounded to mean "Chanayah," camping. Camping down indicates anything fixed, so if he owns a tree this is also fixed.

6) It seems to me that according to the Me'iri's reason, one may also not sell them a stop-sign or a merry-go-round because these are both Mechubar l'Karka and are like Karka, so they would be included in the Gezeirah. According to the Ran also, these items possess a kind of Kevi'us so one may not sell them. The question would be, what will the Chazon Ish say about selling thee items? There is no Yenikah -- they do not absorb sustenance from the ground, so according to this it might be permitted to sell them. But I think we would have to look in other passages in the Chazon Ish's Sefer to see if he gives different reasons for not selling a tree, and then we might be able to deduce more accurately what his opinion would be about selling the stop-sign or the merry-go-round.

7) Let us conclude by discussing the explanation of Rashi on 20b. The Gemara (20b) says that the problem with selling a tree is that the Akum "Meshahi Lei," he lets the tree remain in the ground for an extended period of time. Rashi (DH Meshahi) writes that it therefore transpires that the Yisrael has granted to the Akum "Yishuv and Chanayah in the land," settlement and camping down in the land. We learn from Rashi that the reason one may not sell a tree is because this way the purchaser becomes settled in the land. So even though one did not sell him the land beneath the tree, nevertheless the owner of the tree has gained a holding in the land. It seems that this is similar to what the Ran writes, that one may not give him any kind of permanent status in the land. According to this, if he owns any kind of fixed item in the ground, not only a tree, it would be considered that he is creating a camp there.

8) I would like to add support for what I wrote above from the words of the Chidushei ha'Ra'ah (a Rishon, Rabeinu Aharon ha'Levi, whom many say was also the author of the Sefer ha'Chinuch) to Avodah Zarah 20a.

He writes: "One may not sell them 'Shum Davar' that is attached to the ground." The Ra'ah added the words "Shum Davar" which are not mentioned in the Mishnah. I argue that this means that the Ra'ah is stressing that one may not sell anything that is Mechubar l'Karka, not only something that derives its nourishment from the land, but also totally inaminate objects.

The Ra'ah continues and writes that this is derived from the word "Techanem" which indicates "Chanayah" -- camping down, implying that one must not give them any "Keva," anything fixed. This is almost exactly the same as what the Ran wrote, as I cited earlier. The Ra'ah lived before the Ran, so it is likely that the source of the Ran's explanation is from the Ra'ah.

The idea is that one must not give them "anything permanent" in Eretz Yisrael because this will grant them a holding. Now, the Shulchan Aruch (YD 151:7) writes: "One must not sell to them in Eretz Yisrael anything attached to the ground, for instance a tree or standing corn."

A tree and standing corn are mentioned by the Gemara 20b, but I now argue that we learn from the Ra'ah that these are only two examples, while in fact any fixed item must not be sold to them, so this includes the stop-sign and the merry-go-round.

Dovid Bloom