starting TIPS ladder in Roth

[Gone4Good]

It doesn't matter if you buy at auction (no transaction fee at schwab, fidelity etc.) or on the secondary market.

Although auctions usually clear at a premium yield to where secondary's trade. And, as someone else already stated, there is a big difference in the risk of TIPS where the principal has a zero inflation adjustment (typically new issue) and ones where the inflation adjustment is large (older TIPS). I think auction is the way to go, generally. But certainly there are instances where a secondary trade might make sense too.

And don't confuse "par" with the inflation factor. The price of the bond can be at par, and yet the principal could still be adjusted upward significantly for inflation. The prices you see quoted online and elsewhere typically don't include the inflation factor.