Quick answer
Quantities take the genitive: кило́ помидо́ров. Ask to taste with Мо́жно попро́бовать? — at a market that is normal, not cheeky.
The place
What is on sale
Doing the deal
At the stall
Tasting is expected
At a market, sellers offer slices of melon, spoons of honey and a pinch of nuts without being asked. Refusing is fine, but trying is the normal way to choose — and it opens the conversation that may get you a better price.
Haggling, carefully
Markets are not bazaars: prices are mostly fixed, but a polite А деше́вле мо́жно? at the end of the day, or when you buy several kilos, often works. Buying a round number helps: Возьму́ три кило́, сде́лайте ски́дку.
Typical mistakes
кило́ помидо́ры
кило́ помидо́ров
A quantity takes the genitive plural.
Я хочу́ попро́бовать?
Мо́жно попро́бовать?
The impersonal мо́жно is the polite form of a request.
Check yourself
How do you ask to taste something?