Do not ever pay any kind of token advance or deposit money before any transaction is made with the investor. No genuine investor/buyer would ask for such a token advance. However, if an advisor is assisting you with a transaction and spending time and effort she/he may charge upfront retainer fee. You should be aware that typically such retainer fee is not conditional on success of the deal. Only if you are completely sure of the advisors prior track record and credentials should you pay such retainer fees.
You could also review best practices for business owners and investors on SMERGERS.