Get rid of both seats, they are both expired and need to be destroyed. Instead of just tossing them in hte trash, cut the harness, remove the covers, and write 'DO NOT USE' in black marker on the plastic.
car seats generally expire 6yrs from the date of manufacture. If you are in a bind and need a new car seat, the Cosco Scenera fits infants really well and will rear face t o35lbs and forward face to 40lbs, and most kids can use it until age 3/4. Its $50 at Wal Mart, good buy if you need a basics car seat that will last you 3/4yrs before your child outgrows it.
Its not illegal to use a used car seat, but its dangerous as heck. you have no idea if that car seat was in an accident, how it was treated, if the harness was submerged in water or put in the washing machine (that breaks the fibers making it unsafe) or if it were damaged on an airplane from being checked, etc. Its expired anyway and I'm sure way beyond being safe. Unless you trust yoru childs life iwth the person you bought the seat from, get rid of it.
www.safekids.org and find the car seat safety, and then click 'find an event inyour area' to talk with a CPST, someone who is specifically car seat trained who can answer your questions and hlep you get a car seat for cheap if you need one. PLEASE don't use used seats unless you trust the person you got it from with your childs life. I doubt you do if you got it form a garage sale or a used shop/second hand shop.... those people are only out ot make a buck and don't care about your child. Its a dangerous practice. A new seat is easily afforded and 100% safer.