If you dont have any regrettable sales, you havent been collecting very long (or have ton of disposable money!)
Off the top of my head here's my top 5 I wish i had back! (i made off very good with these sales at the time but if i kept now.. YIKES!) - But without these moves I dont have the collection I do now, which I am so happy and grateful for!
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All of these took place about 6 or 7 years ago
#1 FF5 7.0 = $4500 (It hurt mailing this one out. NEVER wanted to sell, but I really needed the money.)
#2 XM 1 8.0 = $12,500
#3 Hulk 181 9.6 = $4600
#4 FF1 3.0 = $3200
#5 GSXM1 8.5 SS Stan - $850