Do I have to check the source of funds on the seller side too?

As a rule not in the same form as for the buyer, since the seller receives money rather than spending it — but there is no blanket exemption. The seller is subject to the full KYC review (identity, UBO, PEP, sanctions) and to ongoing monitoring; whether and how deeply the origin of funds or wealth must be checked depends on the actual risk. With communities of heirs, or equalisation payments between them, source-of-funds questions regularly become relevant.

The origen de fondos check is, on standard transactions, a plausibility assessment; formal evidence is required by law primarily where risk is elevated (EDD) (Arts. 6, 11 Ley 10/2010). For the seller side that means:

• No origin check on the sale proceeds: with the seller there are no funds whose origin needs checking — they receive the price rather than paying it. The origin of the buyer's money is checked on the buyer side anyway.
• KYC remains mandatory: name, address, ID, UBO (for companies), PEP status and a sanctions-list check must be done for the seller too.
• The community-of-heirs exception: where one heir buys out the others, money moves and its origin must be checked — the paying heir does spend money.
• The judicial auction exception: here the seller may be an institution (a court, a creditor) with KYC processes of its own.

Legal basis: Ley 10/2010, Art. 6

Practical tip: Many agents neglect seller KYC — a mistake, because SEPBLAC looks at both sides in its inspections. Run seller and buyer KYC in parallel.