Japanese armours can be expensive and there are many unscrupulous sellers waiting to relieve you of your money.
By and large the armours, or their components, offered for sale by Japanese dealers are genuine but they are frequently refurbished with new textiles and new leather components. This is perfectly legitimate, after all armours needed refurbishing during their working life, but the price should reflect that they have been worked on. It is rare for old fabrics to survive without some indication of age. A common area for damage to occur is the inner bend of the elbow. If you cannot see any tears, stains or splits then the fabric may well have been replaced. Ask the seller if any work has been carried out. The commonest area for damage to the lacing is just below the bottom of the body section where the pendant kusazuri or gessan have been folded into the body for storage.
Another common practice is to assemble unrelated components, sometimes relacing them to match other elements. Look for differences in the exact colour of the lacing, the shape of scales or the finish on the plates. If they occur they may indicate a different origin. This is not always case since many armours often contained re-cycled elements when they were originally assembled, this being particularly true of helmet bowls.
Another trap for the unwary is the sale of modern repoductions of armours, made in Japan but offered for sale without any indication of age. These can be deceptive but a give-away is that they are laced in cotton braid that has a slightly dull fuzzy appearance and are generally rather over embellished with gilded fittings. On the plainer armour of this genre, the rivets used to hold the body section together show under the thin coat of spray paint. On masks what look like rivets are spot welds whilst the interior is simply sprayed with red paint whereas on a real mask the interior was filled to a smooth finish before the coats of red lacquer were applied.
Sadly there are appearing in ever increasing numbers fake armours from China. Many are ludicrously bad copies that would not fool anyone who has looked at real Japanese armour. A newer and more deceptive trend is for Chinese vendors to copy illustrations from Japanese and other museum catalogues, claiming that they have the armour for sale. What often gives them away is that there will only be the one photograph of the armour, sometimes repeated, beautifully displayed and lit. Just occasionally there might be two views of the armour if they occured in the catalogue. The text generally includes references to Chinese art without mentioning the armour except in vague terms of it being strong, warlike and so on.
If these sellers have more that one entry it is noticable that the background to the photographs will generally be different but the price will be the same irrespective of the quality of the armour. It would pay to request additional photographs of any armour offered for sale from these unscrupulous vendors.
I hope these very few brief notes are a help and I hope you manage to acquire good real pieces.