Продавец сказал, что круглый стол удобнее для семьи, но мне кажется, что прямоугольный практичнее.

Russian has no articles. So продавец can mean a seller or the seller, and стол can mean a table or the table. Context tells you which is meant.

In this sentence, English would usually say The seller said... because we are probably talking about a specific seller in a store.

Сказал is the past tense masculine singular form of сказать.

In Russian past tense, the verb agrees with the subject in gender and number:

• он сказал — he said
• она сказала — she said
• они сказали — they said

Since продавец is grammatically masculine, the sentence uses сказал. If the seller were clearly female, you might see продавщица сказала or sometimes продавец сказала in real-life speech if the speaker is referring to a woman, though the first option is cleaner for learners.

Here что means that and introduces a subordinate clause.

So:

• Продавец сказал, что... = The seller said that...
• мне кажется, что... = it seems to me that...

Russian uses что very often in places where English may use that or may leave that out completely.

For example, in English you can say:

• The seller said the round table is better...

But in Russian, keeping что is normal and expected:

• Продавец сказал, что круглый стол...

Because Russian often omits a noun when it is obvious from context.

The first time, the full phrase is given:

• круглый стол — round table

The second time, стол is understood, so only the adjective remains:

• прямоугольный = прямоугольный стол

This works in English too sometimes:

• The round table is better, but I think the rectangular one is more practical.

In Russian, the adjective simply stands on its own when the noun is clear.

They already are the comparative forms.

• удобный — convenient / comfortable

• удобнее — more convenient / more comfortable

• практичный — practical
• практичнее — more practical

Russian often forms comparatives with endings like -ее / -ей / -e rather than using a separate word like English more.

These comparative forms do not change for gender, number, or case in this use:

• стол удобнее
• кровать удобнее
• эти стулья удобнее

You can also sometimes use более удобный or более практичный, but удобнее and практичнее sound very natural here.

Семьи is the genitive singular of семья.

That is because the preposition для requires the genitive case:

• для семьи — for the family
• для мамы — for mom
• для детей — for children

So удобнее для семьи means more convenient for the family.

Мне кажется literally means it seems to me.

The person experiencing the impression goes in the dative case:

• мне кажется — it seems to me
• ему кажется — it seems to him
• нам кажется — it seems to us

This is a very common Russian way to soften an opinion. It can sound less direct than я думаю (I think).

So:

• я думаю = I think
• мне кажется = it seems to me / I feel / I would say

In this sentence, мне кажется sounds natural because the speaker is politely giving a personal opinion that differs from the seller’s.

Because казаться normally works with the experiencer in the dative:

• Мне кажется... — It seems to me...
• Тебе кажется... — It seems to you...
• Им кажется... — It seems to them...

So мне is the dative form of я.

This is similar to other Russian patterns where English uses a subject, but Russian uses a dative experiencer:

• мне нравится — I like
• мне холодно — I am cold
• мне кажется — it seems to me

In present-tense Russian, the verb to be is usually omitted.

So English:

• The round table is more convenient
• The rectangular one is more practical

becomes Russian:

• круглый стол удобнее
• прямоугольный практичнее

This is completely normal. Russian does use forms of быть in the past and future, but not usually in simple present-tense statements like this.

Russian punctuation requires commas before subordinate clauses introduced by что, and before но.

So the commas are there because of the structure:

• Продавец сказал, что...
• ..., но...
• мне кажется, что...

That gives:

Продавец сказал, что круглый стол удобнее для семьи, но мне кажется, что прямоугольный практичнее.

The commas are not optional here.

They are related, but not identical.

• удобнее focuses on comfort, convenience, ease of use
• практичнее focuses on practicality, usefulness, suitability in real life

So the sentence suggests a contrast like this:

• a round table may be more convenient for a family
• a rectangular one may be more practical

That could mean easier to place in a room, better use of space, easier against a wall, and so on.

Yes. That would be fully correct.

• что прямоугольный практичнее
• что прямоугольный стол практичнее

Both are grammatical. The version without стол is just more natural because the noun was already mentioned earlier.

Russian often avoids repeating nouns when the meaning is obvious.