OK i know a guy selling a M39 this will be my first Mosin what do i need to look for?
Thanks for the help.

What year? That will have an impact. Straight stock, bore condition and original finish are big pluses, as well. Matching numbers are a plus, but mistmatched or "Finn Matched" are not uncommon on Finn rifles, so it's not as big of a detractor if they don't match. Any chance you could post pics and price?

1943 dated receiver; box “SA” some kind of cog circled “S”

A sako crown looking stamp on the stock, and an S with crossed swords.

He is asking $350

I go with bore condition and overall looks as well as matching numbers. If any of the 3 is lacking there are more out there.

You can check it's rarity rating on 7.62x54r.net,  but offhand I would say that is a bit high, depending on bore condition and amount of bluing. Best place to check is Pat Burn's site, see if there are any others with about the same condition going for that price.
You are at a bit of a disadvantage, you do not have many others to compare bore condition to, so you may not be able to see if it is good, very good, or excellent.

Pat currently has a '42 with the following description:

"75% original blue, bright sharp shiny bore, matching bolt & floor plate, partial arsenal tag and a sako war time stock in excellent condition."

Price is $279 on that one, but I think even that's a little high for one that's 75% with nothing really special about it. 1943 isn't  a rare year. It's probably one of, if the the most common years of Sako M39s. The bores on M39s are usually pretty nice, if not like new. If it were anything less than mint, I would pass at $350. I agree that there are plenty of them out there. Again, more details and pics would be required, but that's my opinion with the information that's been given.