Mä tykkään siitä, että kirpputorilla voi jutella myyjien kanssa ja ne kertovat vaatteiden tarinoita.

Mä is the common spoken (colloquial) form of minä (I).

• Minä tykkään = standard / written Finnish
• Mä tykkään = everyday spoken Finnish

In informal speech almost everyone says mä, but in formal writing or exams you should use minä.

The verb tykätä (to like) normally takes the elative case (-sta/‑stä) in Finnish.

• tykätä + elative:
  
  • Tykkään siitä. = I like it / that.
  • Tykkään suklaasta. = I like chocolate.

So:

• se (basic form) → siitä (elative form: from that / about that)
• You cannot say tykkään se; it must be tykkään siitä.

Siitä, että … is a very common way to say I like that… / I’m happy that… etc.

The pattern is:

• Verb + siitä, että
  • full clause

In this sentence:

• Mä tykkään siitä, että kirpputorilla voi jutella…
  = I like it that / I like the fact that at the flea market you can chat…

Siitä is a pronoun referring forward to the whole että-clause.
Literally: I like from-that, that at the flea market one can chat…

Finnish punctuation rules require a comma before most että-clauses, because että introduces a subordinate clause.

So:

• Mä tykkään siitä, että …
  main clause: Mä tykkään siitä
  subordinate clause: että kirpputorilla voi jutella…

Even when English wouldn’t always use a comma, Finnish usually does here.

• kirpputori = flea market
• kirpputorilla = at the flea market

The ending -lla / -llä marks a location on / at something (adessive case).

Examples:

• pöytä → pöydällä = on the table
• asema → asemalla = at the station
• kirpputori → kirpputorilla = at the flea market

So kirpputorilla voi jutella… = at the flea market one can chat…

Here voi jutella uses the impersonal / generic construction:

• (X) voi tehdä jotain = one can do something / you can do something (in general)

There is no explicit subject; it means:

• Kirpputorilla voi jutella myyjien kanssa
  ≈ At the flea market you can chat with the sellers (people generally can).

This is very common in Finnish when talking about what is generally possible.

Both relate to speaking, but:

• puhua = to speak, to talk (neutral, also for languages)
  
  • Puhun suomea. = I speak Finnish.
• jutella = to chat, to have a casual conversation (more informal, relaxed)
  
  • Jutellaan vähän. = Let’s have a little chat.

So voi jutella myyjien kanssa emphasizes friendly, informal chatting with the sellers.

The postposition kanssa (with) requires the genitive case before it.

• myyjä = seller
• myyjien = genitive plural = of the sellers
• myyjien kanssa = with the sellers

Pattern:

• ystävä → ystävän kanssa = with a friend
• lapset → lasten kanssa = with the children
• myyjät → myyjien kanssa = with the sellers

You are right according to standard Finnish:

• he = they (for people)
• ne = they / those (for things)

But in spoken Finnish, people very often use:

• se instead of hän (he / she)
• ne instead of he (they)

So in colloquial speech:

• Ne kertovat vaatteiden tarinoita.
  = He kertovat vaatteiden tarinoita. (standard)
  = They tell the stories of the clothes.

In formal writing, use hän / he. In everyday speech, you will constantly hear se / ne for people.

Kertovat is the they form of the verb kertoa (to tell).

• Verb: kertoa = to tell
• Stem: kerto-
• 3rd person plural: he kertovat / ne kertovat = they tell

Other forms for comparison:

• minä kerron = I tell
• sinä kerrot = you (sg) tell
• hän kertoo = he / she tells
• me kerromme = we tell
• te kerrotte = you (pl) tell
• he kertovat = they tell

Two parts:

1. vaatteiden

   • vaate = piece of clothing, garment
   • vaate → vaatteiden = genitive plural = of the clothes
   • This shows possession / relation: stories *of the clothes*.

2. tarinoita

   • tarina = story
   • tarinoita = partitive plural
   • As the object of kertoa here, the partitive plural is used for an indefinite plural: stories (some stories, not a fixed set).

So:

• vaatteiden tarinoita ≈ stories of the clothes / the clothes’ stories in an indefinite sense.

Yes, you can say vaatteiden tarinat, but the nuance changes:

• vaatteiden tarinoita (partitive plural)
  → focuses on some stories, stories in general, not a complete, fixed set.

• vaatteiden tarinat (nominative plural)
  → sounds more like the specific stories of the clothes, the whole set of them.

In this sentence, tarinoita fits better because we’re talking about the sellers telling various stories in a general, non-limited way.

The structure is:

• …voi jutella myyjien kanssa ja ne kertovat…

So:

• In the first part, myyjien kanssa = with the sellers (a companion, not a subject).
• In the second part, ne refers back to those sellers and is the subject: and they tell…

In more standard written style you might see:

• …voi jutella myyjien kanssa, ja he kertovat vaatteiden tarinoita.

Repeating the noun as subject would be possible but clumsy:

• …voi jutella myyjien kanssa ja myyjät kertovat…
  (grammatical, but less natural than using he / ne here)