На рынке можно купить сколько угодно яблок.

На рынке means at the market or in the market.

The noun рынок becomes рынке because it is in the prepositional case after на when talking about location:

• рынок = market
• на рынке = at the market / in the market

Compare:

• на рынок = to the market (motion toward, accusative)
• на рынке = at the market (location, prepositional)

So here it is about where the buying can happen, not movement toward the market.

Russian often uses impersonal constructions where English would use something like you can, one can, or it is possible to.

Here, можно купить literally means something like:

• it is possible to buy
• one can buy
• you can buy

There is no need to say you, we, or people. The sentence is general and not aimed at one specific person.

Можно means it is possible, one may, or can in a general sense.

It is used in impersonal sentences:

• Можно войти? = May I come in? / Can I come in?
• Здесь можно курить. = It is allowed to smoke here.
• На рынке можно купить яблоки. = You can buy apples at the market.

In your sentence, можно does not mean physical ability like to be able to in a personal sense. It expresses general possibility or availability.

This is about aspect.

• купить = perfective, to buy / to make a purchase, viewed as a complete act
• покупать = imperfective, to be buying / to buy in general / repeatedly

After можно, both aspects are possible, but they give slightly different meanings.

• можно купить = it is possible to buy, to obtain, to purchase successfully
• можно покупать = one may buy / one is allowed to buy / one can be buying, often more process-oriented or habitual

In На рынке можно купить сколько угодно яблок, купить sounds natural because the idea is that the market is a place where apples can be obtained in any quantity.

Сколько угодно is a fixed expression meaning:

• as many as you want
• any amount you like
• however many you want

It is very common in Russian.

Examples:

• Ешь сколько угодно. = Eat as much as you want.
• Можно взять сколько угодно книг. = You can take as many books as you want.

So in your sentence, сколько угодно яблок means as many apples as you want.

Because after сколько угодно, Russian normally uses the genitive plural.

• nominative plural: яблоки = apples
• genitive plural: яблок = of apples

Russian uses the genitive plural after many quantity expressions, including:

• много яблок = many apples
• мало яблок = few apples
• сколько яблок? = how many apples?
• сколько угодно яблок = as many apples as you want

So яблок is required here because the phrase is about quantity.

Yes, На рынке можно купить яблоки is also grammatical, but it means something slightly different.

• На рынке можно купить яблоки. = You can buy apples at the market.
• На рынке можно купить сколько угодно яблок. = You can buy as many apples as you want at the market.

The second sentence emphasizes unlimited quantity or no practical limit.

Also, with a quantity expression like сколько угодно, the noun must be in genitive plural: яблок.

Yes, Russian word order is flexible, though each version may shift the emphasis.

The neutral order here is:

• На рынке можно купить сколько угодно яблок.

This puts the setting first: at the market.

Other possible orders:

• Сколько угодно яблок можно купить на рынке.
• Яблок сколько угодно можно купить на рынке.
• Можно купить сколько угодно яблок на рынке.

These may sound more marked or place emphasis in different places. For example:

• starting with сколько угодно яблок emphasizes the large quantity
• starting with на рынке emphasizes the location

So the original sentence is a natural, neutral way to present the idea.

Because можно is not a normal verb form that agrees with a subject. It is a predicative word used in impersonal sentences.

So it stays the same regardless of who is involved:

• Мне можно? = Am I allowed? / May I?
• Здесь можно купить хлеб. = You can buy bread here.
• Нам можно войти? = May we come in?

It does not change for person, number, or gender.

It is generally neutral and very common in both spoken and written Russian.

You can use it in everyday speech:

• Бери сколько угодно. = Take as much as you want.

And also in more standard written language:

• Посетители могут выбрать сколько угодно товаров. = Visitors may choose as many items as they want.

So it is a very useful phrase to learn.

A very literal breakdown would be:

• На рынке = at the market
• можно = it is possible / one can
• купить = buy
• сколько угодно = as many as desired / however many you want
• яблок = of apples

So the structure is roughly:

At the market, it is possible to buy however many apples one wants.

That is more literal than natural English, but it shows how the Russian sentence is built.