I recently sold a truck and did not write "sold as it" on the bill of sale, do i have to return the buyers money

It's not that simple.  First, although you've written nothing about it above, I assume that there turned out to be some defect in the truck that would normally allow the buyer to rescind the sale.  I also assume that you are not in the business of selling trucks, and the UCC implied warranties therefore don't apply to this sale. Writing "as is" would be excellent evidence that the agreement was for an "as is" sale, but the deal could still have been "as is" even though you didn't write it on the bill of sale, if, in fact, that's what you agreed to (such an agreement could possibly be implied from the circumstances of the sale if this issue was never mentioned.)  The problems are that (a) if the bill of sale contains all the terms of a normal contract, it could be deemed a fully integrated written agreement and you would not be able to testify to any oral terms that contradict the writing (based on the little you've written, this issue could be decided either way); and (b) the buyer may claim that it was not an "as is" sale, and from what you've written, it is his word against yours.