In my opinion NYStamps has a very poor reputation and state right in their Terms and Conditions that some stamps "may be expertly repaired". If you have previously purchased from them single, higher value stamps from them and NOT gotten them certified then you will likely be disappointed if you do or when you go to dispose of your collection(s).
You can search this forum and many threads about this seller. There are some good sellers on eBay
including many who are also members here. Learning how to find quality sellers is as hard as learning how to spot quality stamps. eBay
feedback is meaningless, some of the worse sellers have great feedback score ( eBay
changes the feedbacks for them, you can go buy existing eBay
accounts, you can pump up your own feedback score at will, etc.)
Finding good value on eBay
is not 'sorting by lowest price first', it is about spending time and money finding good sellers and supporting them. And NYStamps is about as far from a good seller as you can get. Try kenr2, fairdinkumstamps, revenue-collector, deyes2, paperhistory, or many of the other SCF members, most are quality eBay