Yes, you give thanks TO someone.
"I thank to the customer" doesn't work in English. The English phrase would be "I give thanks to the customer". I wouldn't use this though. "Giving thanks" isn't used in daily speech and seems to only be used in specific contexts, usually religious ones. I only mentioned "give thanks to" to help people remember that it is dative case.
I would suggest sticking with one of these two:
- I thank the customer.
- I am thanking the customer.
"I give thanks to the customer" was not accepted. It is, as you say only used in specific cases in english, but usually duolingo takes literal translations and also provides the best english equivalent. Also, the german word translates as "consumer". The customer is not necessarily the consumer. For example, the retailer may be the customer of the wholesaler or producer and will re-sell to the ultimate consumer of the food substance/utility service. But maybe Verbraucher is colloquial german for customer in a region?
Yes; as the article I linked to above says, there are three general meanings of the inseperable prefix ver-:
ver- Inseparable verbal prefix for- vergeben, 'to forgive'
Inseparable verbal prefix that denotes a transition of the object into a state, which is indicated by the stem. lieben 'to love' → verlieben 'to fall in love'
Inseparable verbal prefix indicating a negative action of the stem. laufen 'to walk → verlaufen 'to get lost'
Kaufen-verkaufen is clearly the last. Brauchen-verbrauchen seems to have wandered a bit further.
We used to have the same prefix in English, spelled "for": it survives in "forlorn" meaning "totally bereft" & "fordone" meaning "totally finished" -- i.e. exhausted. But you used to be able to slap it in front of any verb, as you still can in German, to express doing the action of the verb to the point of destruction.