When buying Target cards from Gift Card Mall on eBay, it is most cost efficient to buy the least number of the same denomination cards. GCM sells Target cards in $50/100/150 denominations. For example, for ~$1000, you should buy either 6x150 ($900) or 7x150 ($1050), rather than 10x100 ($1000) or 6x150 + 1x100 ($1000).
The reason is you need to think about minimizing your shipping costs, which eats into your profit. GCM charges $2.99 shipping for the first card of any denomination and $0.99 for each additional card of the same denomination.
So for the above examples, shipping charges would be $7.94 for 6x150, $8.93 for 7x150, $11.90 for 10x100, and $10.93 for 6x150 + 1x100. When spending Target cards, it's also to your benefit to have higher denomination cards because Target limits the number of gift cards per transaction to 4.
@DIS_MERI has been keeping track of when eBay Bucks promos have been offered, which should give you an idea about the frequency, and each promo usually lasts 2 and very rarely 3 days: