nds-card.com? They were GBAtemp's sponsor for a long time, made several purchases from them myself and so did many others on this site. Costello & co wouldn't promote any flashcart store that was not serious about their business. I know this from experience, interestingly, as Costello used to run a flashcart price comparison website as a side business (stores had to pay a small fee to be listed there), I bought from one of the sellers, they never shipped and wouldn't respond to emails. A quick message to Costello and the next day they emailed me apologizing and asking me to please not get them delisted from the site.
nds-card
s.com would be an entirely different site though, not familiar with them.
Anyways, I can't speak for how nds-card is nowadays, but it looks like it's still the same store. I just lost interest in them when they ran out of R4i Gold (r4ids.cn) carts since they had the best price on that for the longest time and it was why I kept recommending them to everyone.
That guy is full of crap though, I can't deny many of these stores are shady, but they usually aren't scams. There are more efficient ways to scam people than selling $10 flashcarts