Buying second hand tapes is ok if you can see or know how much they were played. Sometimes they show little use and can be considered virtually new when properly degaussed. However, getting them and discovering splices, silent sections (tape without oxyde) or even wear from numerous plays can be very disappointing.
Two years ago I got over 250 7" reels. About 60% of them were Ampex 456. Several started to drop out, some others produced hisses on my Nagra tension rollers so I just trashed all of them. All the rest (40%) are Maxell XL-35. I couldn't look into the boxes so I took the lot as is for quite cheap ($1.50 a tape). Later I discovered that both brands have partially empty reels, some with silent tape. Fortunately they are not many and in any case, those tapes were used for mastering, and had almost no passes, or VERY few.
Another thing I want to tell you: here in France, radios used PER-528 tape a lot. This tape works great and is still made by RMGI. It's not very expensive and you would probably like it a lot. I can't remember the correct reference but I think it's still PER-528. Check RMGI's site for more infos.
Personally I have two RMGI LPR-35 reels. Man, they really sound nice. I was told that SM-900 are even better. At least, LPR35 gives you 90 minutes at 7.5ips on a 10.5" reel, as opposed to the 60 minutes of the PER-528.
David.