im getting a used helix downpipe/ high flow cat and greedy catback, the guy says he has no seals for it but i can use my stock seals. is this true it seems like the stock gaskets would have a smaller inner diamiter than the 3". if i cant use the stock ones what all gaskets would i need to get? the cheeper the better for me

 

Im not exactly sure which ones you'll need but i know you'll need to get new ones!

 

you just need some standard two hole flange exhaust gaskets (3"). lots of places sell them for not too much. try searching some different vendors.

 

I did the same thing, purchased a used TBE exhaust and bought new O2 housing.
The O2 housing to the DP I reused the OEM donut gasket.
For the DP to cat and cat to cat back I actually didnt use gaskets. I used RTV high temperature gasket maker. It has been successful months and months later with no leaks or nothing.

If you want to purchase new gaskets you will need the size of the exhaust. I'm assuming your exhaust is 3in so you would need 3in gaskets. They are available at most auto stores.

I did this same search and the majority of the people suggested RTV. New gaskets are a good idea, but I wanted to input my experience which is the same as yours. Hope this helps.