I have an argon laser. It depends on the asking price. If it is local, I wouldn't pay more than $100 for it if it is still working well and isn't long on hours. My single line argon produces much more power than the rated 20 mW. You might want to see how much power it is able to produce. Check the time meter on it also.
You could ask a trusted member to 'middle' this buy--
You pay the middle and he gets the delivery and-- sends all after a test-- or you pay the 'middle' and the laser is shipped to you-- if you do not like you ship it back and your 'middle' sends your $$ back once it is returned or you keep it and the middle sends the funds.
I have never used this method but have been a 'trusted' member in the 'middle'. It may be helpful if the 'middle' in near the seller--less ship costs that way-- even better - if you can meet up with the seller and no 'middle' is needed.
Pretty fool proof.
only neg is sometimes there is an additional ship cost.