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welcome to AFF
Be very wary - many of these sites obtain flights using FF points/miles and onsell them which is against the T&C's of any real loyalty program I am aware of - if such is discovered it is likely your flights would be cancelled - you might find out so as late as checkin.
What web sites were you considering?
I too would be wary, I've been approached about using them for clients flights but I wouldn't touch them, just not worth it.
As the saying goes, if it seems too good to be true.....
TG