All three sentences are in practice perfectly acceptable. The only objection is likely to come from the seller who thinks that the laptop was OK when it was sold or that it was someone else who should be blamed. They will not be worrying about the grammar.
If you were being really pernickety you might say that 'from' is not correct because the laptop was purchased from the seller not from the store.
Likewise, bearing in mind that in the UK, at least, multiple vendors of laptops might operate in a single store, if you say 'in' then you may not be writing to the right person.
I can see pernickety objections to 'at', but really if you say you bought the laptop at Walmart no-one will misunderstand you or think you lack a grasp of the English language.