Providing proper documentation (sales contract or agreement) to sell your property is the responsibility of your agent. Of course you will need to have access to all paperwork relevant in this case (house registration book, company registration documents, land title deeds, annual maintenance fees, receipts, manuals, warrantee documents, copy of passport(s) etc).
Professional agents will conduct a legal evaluation of all documents avoiding that there is anything missing or corrupt which could obstruct a transfer without any problem. Experienced realtors are familiar with all rules and regulations of the Land Office Department which will become obviously if they would be handling your transfer.
Less experienced agents could cause serious problems if they don’t know what they are doing. Please be reminded that you are still in Thailand.