Which is right? “… purchased from/in/at your store”

All three sentences are in practice completely appropriate. The only objection is most likely to come from the seller who believes that the laptop computer was OK when it was offered or that it was another person who ought to be blamed. They will not be fretting about the grammar.

If you were being really pernickety you might say that ‘from' is not appropriate due to the fact that the laptop was bought from the seller not from the shop.

Also, keeping in mind that in the UK, at least, several vendors of laptops may run in a single shop, if you state ‘in' then you may not be writing to the right person.

I can see pernickety objections to ‘at', but really if you say you purchased the laptop computer at Walmart no-one will misunderstand you or think you do not have a grasp of the English language.

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